[FEATURE]: Rename ClientFunction to AsynchronousClientFunction in Flutter renderer

[redbrogdon]

• I have searched the existing issues to make sure this feature has not already been requested.

Is your feature request related to a problem? Please describe.

In the Flutter rendered, client-side functions come in two varieties, synchronous and asynchronous. Because of the way they were implemented, the names ended up as SynchronousClientFunction and ClientFunction.

Describe the Proposed Solution

While they're perfectly usable this way, I believe it would be easier for new devs to adopt the API if they were renamed to SynchronousClientFunction and AsynchronousClientFunction (or Sync- and Async-).

Describe Alternatives Considered

The main alternative would be to leave them as is, though I think devs are used to seeing Sync/Async naming conventions.

[polina-c]

Gave P2 to it to take out of triage queue. Feel free to change priority.