Adds swift FML code generation

[tarikeshaq]

fixes EXP-1984

• Allows running swift scripts against FML generated code
• Allows running swift script against a mock nimbus feature variables API
• Generate swift FML code similar to how Kotlin does it
• Write tests to cover at minimum what the Kotlin tests cover
• Clean up FeatureVariables API so UIKit is imported again
• Have the "real" nimbus API use `FeatureInterface"
• Clean up code and TODOs in comments
• Make unit tests pass
• Rebase
• Test against Firefox iOS to make sure there are not breaking changes
• Changelog

[codecov-commenter]

Codecov Report

Merging #4780 (fb40792) into main (134c50e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4780   +/-   ##
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  Coverage   81.49%   81.49%           
=======================================
  Files          49       49           
  Lines        5653     5653           
=======================================
  Hits         4607     4607           
  Misses       1046     1046           

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[tarikeshaq]

Early notes, I think I have the testability of this setup nicely. Scripts can run and access the MockNimbus API. I have to think a little about the UIKit dependency, but I can do that later when this PR is in a ready state 🤷

[jhugman]

Small 🚲 🚗 🚜 🚌 Drive By review.

The Rust looks boring and regular; good job; otherwise, I haven't commented much.

I've added a few comments about the Swift. Keep going looks nice.

[travis79]

Drive-by review (I just read James' review and I didn't have much to add beyond his suggestions to make the code a little more "swifty").

[tarikeshaq]

OK tested against firefox-ios locally and verified there are no breaking changes for Nimbus (had to test against the places_uniffication and the glean update patches since those are on main here, but regardless, this patch doesn't introduce a breaking change)

also added the changelog so this should be in a good place for review 😄

[travis79]

Just a handful of minor nits, but this looks fantastic! Well done!

[jhugman]

Lovely.

I've left some comments which should be addressed, but nothing to hold up another review cycle.

[jhugman]

if let is a nil check, so it's redundant here.

Suggested change

	if let s = s {
	return {{class_name}}(rawValue: s)
	}
	return nil
	return {{class_name}}(rawValue: s)

[tarikeshaq]

Yeah, the nil check is intentional here because the enumValue in swift takes a String?, since the v.getString("the-property") returns a String?

[jhugman]

I'm not sure about these kotlin names; in general I've tried to keep naming schemes familiar to the platform, but this is all very internal.

[jhugman]

This is … suprising. I'm not sure I understand why this is necessary.

[tarikeshaq]

This is only for the tests, the reason is that the scripts are run after the FeatureManifest, and all the Nimbus helpers are compiled together into a module that's later imported by the script. In the app, the FeatureManifest would be compiled together with the app so this wouldn't be needed