[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.

Find the Flutter renderer code where ClientFunction and SynchronousClientFunction are defined. Look for their usage across the codebase to understand the scope of the rename. After renaming, ensure all references are updated and run any existing tests for the Flutter renderer to verify the changes.

索引モデルが issue の本文から書いたものです。