[url_launcher] Adds macOS support

[stuartmorgan-g]

Description

Follows the structure established in #2119 to add a federated macOS implementation of the url_launcher plugin.

Related Issues

Fixes macOS portion of flutter/flutter#41721

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Breaking Change

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[stuartmorgan-g]

/cc @franciscojma86

[stuartmorgan-g]

This doesn't include tests, since I'm not sure what the integration test strategy for federated plugins is. Should the example in the main plugin depend on the federated version and include macOS support?

[amirh]

This doesn't include tests, since I'm not sure what the integration test strategy for federated plugins is. Should the example in the main plugin depend on the federated version and include macOS support?

Interesting point, we should probably think about it some more, I think it would be nice if the platform package could provide the example's platform scaffold (e.g for Android that will be the Activity, manifest etc...) and the e2e tests that are specified in the app-facing plugin will be executed as part of the macOS CI. Though devil is in the details, and I'm not sure how we'll end up with a pubspec that has all the example app's dependencies but also a dependency on the unendorsed implementation.

cc @collinjackson

[amirh]

I haven't reviewed the swift code but the structure and docs all make sense.

We probably need some test before this can land, so either we figure out the e2e testing story, or maybe it's possible to come up with a unit test for the swift code?

[collinjackson]

@franciscojma86 once we have driver testing of Mac apps on CI, this PR can land.

[stuartmorgan-g]

Obsoleted by #2383