feat(extension): run separate programs as experimental extensions

[zimeg]

Summary

This PR adds the extension experiment and corresponding command to run other programs as "extensions".

Preview

$ which slack-echo                                  # Find a custom "slack" program exists
/usr/local/bin/slack-echo
$ slack extension exec --experiment extension echo  # Run whatever the "echo" program does
...

Reviewers

$ slack extension --help
$ slack extension exec --experiment extension glitch   # Use an included extension
$ slack extension exec --experiment extension beyonce  # Praise the stars

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.08%. Comparing base (5033493) to head (fb8d122).

Files with missing lines	Patch %	Lines
internal/hooks/hook_executor_default.go	12.50%	6 Missing and 1 partial ⚠️
cmd/extension/exec.go	98.48%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   62.96%   63.08%   +0.11%     
==========================================
  Files         212      214       +2     
  Lines       21782    21881      +99     
==========================================
+ Hits        13716    13804      +88     
- Misses       7005     7014       +9     
- Partials     1061     1063       +2     

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