fix(*): replace deprecated pl.xml module with luaexpat

[windmgc]

Summary

This PR has the following changes:

• Replace deprecated pl.xml module with luaexpat. Currently, the XML parser is only used in TokenFileWebIdentityCredentials.lua. Since STS service is responding with XML response, it is also needed in ChainableTemporaryCredentials.lua. This should solve Dependency on deprecated Penlight XML (pl.xml) module #37 (as long as luaexpat does not get deprecated :P)
• Fix the logic that fetches temporary credentials from STS service, this is necessary for implementing assumeRole-related features.

[windmgc]

By running the original tests I found that Luaexpat's behaviour seems quite different from Penlight's implementation of simple XML parser...

(kong-dev) ubuntu@ip-172-31-93-219:~/kong-ee$ cat test3.lua
local a = [[<?xml version="1.0" encoding="UTF-8"?>
<mainXmlElement xmlns="cool-name-space">
  <someSubStructure>
    <hello>the default hello thinghy</hello>
    <world>the default world thinghy</world>
  </someSubStructure>
  <subList>
    <listELement>1</listELement>
    <listELement>2</listELement>
    <listELement>3</listELement>
  </subList>
  <RoleArn>hello</RoleArn>
  <RoleSessionName>world</RoleSessionName>
</mainXmlElement>
]]

local parser = require("lxp.lom")
local inspect = require("inspect")

print("======LUAEXPAT======")
print(inspect(parser.parse(a)))

print("======PL.XML======")
local pl_parser = require("pl.xml")
print(inspect(pl_parser.parse(a, false, true)))

(kong-dev) ubuntu@ip-172-31-93-219:~/kong-ee$ resty test3.lua                                                                                                                                                                                                          [42/18206]
======LUAEXPAT======
{ "\n  ", { "\n    ", { "the default hello thinghy",
      attr = {},
      tag = "hello"
    }, "\n    ", { "the default world thinghy",
      attr = {},
      tag = "world"
    }, "\n  ",
    attr = {},
    tag = "someSubStructure"
  }, "\n  ", { "\n    ", { "1",
      attr = {},
      tag = "listELement"
    }, "\n    ", { "2",
      attr = {},
      tag = "listELement"
    }, "\n    ", { "3",
      attr = {},
      tag = "listELement"
    }, "\n  ",
    attr = {},
    tag = "subList"
  }, "\n  ", { "hello",
    attr = {},
    tag = "RoleArn"
  }, "\n  ", { "world",
    attr = {},
    tag = "RoleSessionName"
  }, "\n",
  attr = { "xmlns",
    xmlns = "cool-name-space"
  },
  tag = "mainXmlElement"
}
======PL.XML======
{ { { "the default hello thinghy",
      attr = {},
      tag = "hello",
      <metatable> = <1>{
        __index = <table 1>,
        __tostring = <function 1>,
        __type = "doc",
        add_child = <function 2>,
        add_direct_child = <function 3>,
        addtag = <function 4>,
        child_with_name = <function 5>,
        children = <function 6>,
        childtags = <function 7>,
        filter = <function 8>,
        first_childtag = <function 9>,
        get_attribs = <function 10>,
        get_elements_with_name = <function 11>,
        get_text = <function 12>,
        maptags = <function 13>,
        match = <function 14>,
        matching_tags = <function 15>,
        reset = <function 16>,
        set_attrib = <function 17>,
        set_attribs = <function 18>,
        subst = <function 19>,
        text = <function 20>,
        tostring = <function 21>,
        up = <function 22>
      }
    }, { "the default world thinghy",
      attr = {},
      tag = "world",
      <metatable> = <table 1>
    },
    attr = {},
    tag = "someSubStructure",
    <metatable> = <table 1>
  }, { { "1",
      attr = {},
      tag = "listELement",
      <metatable> = <table 1>
    }, { "2",
      attr = {},
      tag = "listELement",
      <metatable> = <table 1>
    }, { "3",
      attr = {},
      tag = "listELement",
      <metatable> = <table 1>
    },
    attr = {},
    tag = "subList",
    <metatable> = <table 1>
  }, { "hello",
    attr = {},
    tag = "RoleArn",
    <metatable> = <table 1>
  }, { "world",
    attr = {},
    tag = "RoleSessionName",
    <metatable> = <table 1>
  },
  attr = {
    xmlns = "cool-name-space"
  },
  tag = "mainXmlElement",
  <metatable> = <table 1>
}

It seems that luaexpat is parsing those new line characters into items which is not expected

[windmgc]

@Tieske Do you think it is a good idea that we remove the newline chars when concatenating the result of poor_mans_xml_encoding? It is not necessary for the XML format after all

lua-resty-aws/src/resty/aws/request/build.lua

Line 236 in 98683b8

request.body = table.concat(xml_data, "\n")

[Tieske]

By running the original tests I found that Luaexpat's behaviour seems quite different from Penlight's implementation of simple XML parser...

Yes, because PL was taking shortcuts, and dismissed whitespace, which is significant in the XML standard. Though in most use cases it is not.

[windmgc]

OpenResty 1.17.8.2's image seems too old to have a libexpat >= 2.3.0, so I changed it to build libexpat from source.