[iOS] Add iOS live text input engine side support

[luckysmg]

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Related PR in flutter framework (WIP)
flutter/flutter#96637

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#91896

This PR will let the framework side query the device live text input ability.(See framework PR classLiveText)

If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• 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 ///).
• I signed the [CLA].
• All existing and new tests are passing.

[jmagman]

LGTM!
cc @LongCatIsLooong re: #29229

[LongCatIsLooong]

LGTM. Any reason for implementing isLiveTextInputAvailable in the platform plugin instead of the text input plugin?

[luckysmg]

Hi @LongCatIsLooong
In my op

1. LiveText is a kind of platform ability (just like clipboard).Maybe there are more functions will be added in the future
   (eg:Maybe like recognizing the text in images on iOS16, that is not much related with text input). So Adding it in FlutterTextInputPlugin may be not so appropriate.

2. Developers may query this value any time even user is not inputing in any textField (So activeView in FlutterTextInputPlugin is nil and we will still create another UITextField to get this value), which is not so elegant.

So I think it is more appropriate to put it in platform plugin.
You can See the framework interface LiveText I wrote in framework PR:
packages/flutter/lib/src/services/live_text.dart

[cyanglaz]

LGTM

[luckysmg]

I merged main to this branch and test passed.
Shall we merge this? @jmagman @cyanglaz @LongCatIsLooong