Add "connect list" command and HTTP library infra

[Cody A. Ray (codyaray)]

@confluentinc/caas

This PR adds the first end-to-end CLI command. It lists all the connectors in an account.

The core is a new HTTP package for our API.

authURL = "https://confluent.cloud"

client := confluent.NewClient(http.DefaultClient, authURL, logger)
token, err := client.Login(email, password)
check(err)

user, err := client.User()
check(err)

client := confluent.NewClientWithJWT(context.Background(), token, authURL, logger)
connectors, _, err := client.Connect.List(user.Account.ID)
check(err)

fmt.Println(connectors)

HTTP Library Details:

• The http package is a standalone wrapper for all API requests & responses.
• Each module has its own service object in the http package (client.Connect.List)
• Flexible authentication: inject oauth2.Token into http.Client instead of hardcoding security mechanism inside http package
• Use https://github.com/dghubble/sling to simplify all the HTTP request/response stuff

Related changes:

• Refactor auth CLI commands to use HTTP library
• A successful confluent login calls /api/me and adds user/account info to config file
• A common CLI error-handling routine is used for standard outputs/responses for each error
• Update connect plugin logs to append instead of truncate each call

Here's example CLI output from various scenarios:

$ go run main.go connect list 
You must login to access Confluent Cloud.

$ go run main.go login --url https://stag.cpdev.cloud
Enter your Confluent Cloud credentials:
Email: cody@confluent.io
Password: 
Logged in as cody@confluent.io

$ go run main.go connect list 
[id:"cp42" name:"Super-\342\233\204" physical_cluster_id:"cp2" ...]
# I'll format outputs and such soon... out of scope for this PR

// Wait til token expires

$ go run main.go connect list
Your access to Confluent Cloud has expired. Please login again.

// After logging in again

$ go run main.go logout
You are now logged out

// Example if you try hacking a token in ~/.confluent/config.hcl.

$ go run main.go connect list 
Error: malformed
Usage:
  confluent connect list [flags]

Flags:
  -h, --help   help for list

exit status 1

// Example if we missed handling an API error

$ go run main.go connect list 
Error: confluent (400): malformed token
Usage:
  confluent connect list [flags]

Flags:
  -h, --help   help for list

exit status 1

[Joshua Buss (chicagobuss)]

awesome, great work!

[Travis Jeffery (travisjeffery)]

+1 on reusing corev1

[Travis Jeffery (travisjeffery)]

👍