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).
⚠️ Report is 363 commits behind head on main.

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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