[google_maps_flutter] Convert remaining code to Swift (#12768)

Converts remaining code to Swift:
- Converts the protocols defined for testability to Swift
  - Where possible, replaces manual passthrough classes with extensions that add protocol conformance to the actual class, reducing the amount of untestable code we need.
- Switches Pigeon generation to Swift (which causes a lot of mechanical changes due to classes no longer having a prefix, and the constructors being standard Swift construction instead of bridging to Obj-C convenience constructors)
- Uses Pigeon's Swift generator support for subclassing to replace the somewhat hacky nested classes in the interface definition with the actual subclassing that we wanted to express, simplifying some code and adding more type safety.
- Scrubs `NSObject` conformance from cases where it's no longer necessary. (Class that implement Flutter plugin API protocols still need it, as those are Obj-C protocols.)

Unlike the other PRs, this does not contain any Gemini Obj-C -> Swift conversions (other than some simple protocols); this is mostly just a Pigeon generation change and a lot of mostly mechanical cleanup.

Fixes https://github.com/flutter/flutter/issues/119108

## Pre-Review Checklist

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