feat(experiment): add apps.icon.set support for non-hosted app icons

[srtaalej]

Changelog

No user-facing changes until apps.icon.set endpoint is live

Summary

This PR adds IconSet() API method targeting the apps.icon.set endpoint for non-hosted (Bolt) app icons. Gated behind the set-icon experiment flag (--experiment=set-icon) (see Note)

Note

🔴 The apps.icon.set endpoint is not yet publicly available. This PR is ready to ship once the endpoint is live. No changes were made to the existing apps.hosted.icon flow.

Required scope (app_configurations:write) may be subject to change before the endpoint goes public

Test plan

slack run --experiment=set-icon with an icon.png uploads icon once endpoint is available
slack run without the experiment flag does not attempt icon upload for non-hosted apps
slack deploy for hosted apps continues to work as before

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 31.57895% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.02%. Comparing base (5ee9e10) to head (4d6a19e).

Files with missing lines	Patch %	Lines
internal/pkg/apps/install.go	0.00%	12 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #469      +/-   ##
==========================================
- Coverage   71.03%   71.02%   -0.02%     
==========================================
  Files         220      220              
  Lines       18522    18539      +17     
==========================================
+ Hits        13158    13168      +10     
- Misses       4185     4190       +5     
- Partials     1179     1181       +2     

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