[google_maps_flutter] Convert remaining code to Swift

[stuartmorgan-g]

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 flutter/flutter#119108

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request converts the remaining Objective-C code to Swift for both the google_maps_flutter_ios_sdk9 and google_maps_flutter_ios_sdk10 packages. This includes updating the Pigeon configuration to generate Swift code, removing Objective-C files, and refactoring Swift implementation and test files to use the new Swift-generated Pigeon APIs. Two minor typos were identified in the changelog files.