Install fish shell completions via pip

[yoshibase]

Summary

Fixes #912.

Install HTTPie's existing Fish completions through Python wheels and source distributions, so Fish discovers completions for the http command without downstream package-specific copying.

Changes

• Add share/fish/vendor_completions.d to setup.cfg data files
• Install the canonical extras/httpie-completion.fish unchanged
• Add a command-named http.fish loader that sources the canonical definitions, avoiding a duplicated completion script
• Include both files in the source distribution manifest

Test plan

• python -m build --sdist --wheel
• Wheel contains share/fish/vendor_completions.d/http.fish
• Wheel contains share/fish/vendor_completions.d/httpie-completion.fish
• Source distribution contains both source files

[yoshibase]

CI note after the clean rebuild: the current PR is only the 3 packaging/completion files, and the wheel + sdist build succeeds with both Fish files in share/fish/vendor_completions.d.

The remaining red jobs reproduce on upstream master (for example run 28435490851): Big5 detection assertions, Python 3.7 unavailable on Ubuntu 24.04, argparse formatting drift, and an unrelated urllib3 digest-auth error. None touches this PR's files.