[Impeller] Support external textures on iOS

[ColdPaleLight]

fix https://github.com/flutter/flutter/issues/112364

1. Introduce Texture::PaintContext to pass parameters.
2. Support BGRA format external textures on iOS.

I have tested this patch locally and it works well.

Note that external textures in YUV format are not yet supported, I will support it in the PR https://github.com/flutter/engine/pull/36685.

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [x] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• [x] I listed at least one issue that this PR fixes in the description above.
• [ ] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the [CLA].
• [x] All existing and new tests are passing.

[chinmaygarde]

Ping @ColdPaleLight. Is there anything to do to unblock making progress on this? We'd really like to work on this as it is a high priority task but don't want to step on your toes :)

[ColdPaleLight]

@chinmaygarde I think this PR is ready for review now :)