chore: use ReleasePlatform type instead of String

[bryanoltman]

Description

Replace stringly-typed concept of platform with ReleasePlatform enum.

Also, we don't need to implement named for enums, as .values.byName exists in dart:core and serves the exact same purpose.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (79da63f) 100.00% compared to head (aff28a2) 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #867    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           78       144    +66     
  Lines         2957      3532   +575     
==========================================
+ Hits          2957      3532   +575     

Impacted Files	Coverage Δ
...horebird_cli/lib/src/code_push_client_wrapper.dart	100.00% <ø> (ø)
...horebird_cli/lib/src/commands/preview_command.dart	100.00% <ø> (ø)
...s/check_for_patches/check_for_patches_request.dart	100.00% <ø> (ø)
..._patch_artifact/create_patch_artifact_request.dart	100.00% <ø> (ø)
...patch_artifact/create_patch_artifact_response.dart	100.00% <ø> (ø)
...ease_artifact/create_release_artifact_request.dart	100.00% <ø> (ø)
...ase_artifact/create_release_artifact_response.dart	100.00% <ø> (ø)
...essages/update_release/update_release_request.dart	100.00% <ø> (ø)
...e_push_protocol/lib/src/models/patch_artifact.dart	100.00% <ø> (ø)
...push_protocol/lib/src/models/release_artifact.dart	100.00% <ø> (ø)
... and 15 more

... and 50 files with indirect coverage changes

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