[FEATURE]: Rename ClientFunction to AsynchronousClientFunction in Flutter renderer

- [X] 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.