[google_maps_flutter] Adopts new async/await Swift Pigeon support (#12860)

Switches from the callback-based Pigeon API to the new async/await-based API, which is:
- more idiomatic modern Swift
- safer, both since it uses Pigeon-generated actor annotations to ensure proper threading, and because it prevents the error case of calling the callback some number of times other than exactly once (which violates the engine API contract)
- the syntax used by the new FFI backend that we plan to switch to in a follow-up

I verified that with this version of Pigeon we are no longer seeing thread violation logging in manual testing (see https://github.com/flutter/flutter/issues/192199)

There is some extra `Task` boilerplate in many callbacks still because we are implementing APIs that are defined by the Google Maps API, and those are not yet using `async` and actor annotations, so we have to bridge them in the plugin layer.

Part of https://github.com/flutter/flutter/issues/192720

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