Add FlTextInputPlugin

[robert-ancell]

Builds on #18220 and #18313

[robert-ancell]

This will fix flutter/flutter#54853 and flutter/flutter#30662.

[stuartmorgan-g]

From a quick skim, my initial feedback (other than we should share a slightly refactored text input model if possible, per Discord discussion, since I would rather we not maintain N implementations of the same embedding-side model indefinitely) is to wonder why things like editing state callback handlers are in FlView. I think the text input "plugin" (poorly named, as it's not an actual plugin) structure that's used on most of the other embeddings (at least iOS, macOS, Windows, and GLFW) is a good separation of concerns. FlView is the kind of thing that can become increasingly large over time if we're not careful.

[robert-ancell]

Looks like I pushed this to the flutter/engine repo not robert-ancell/engine. I've been updating this in the wrong branch. I've pushed all my changes now and this should be ready for review.

[stuartmorgan-g]

LGTM with some minor changes.