[google_maps_flutter] Semi-convert remaining iOS host API calls to Pigeon

[stuartmorgan]

Does a "shallow" Pigeon conversion of the remaining host API calls in the iOS implementation. All of the actual method calls are now Pigeon, but for the most part the data structures are not yet converted, and the Pigeon representations of the map objects are instead placeholders that currently just wrap the existing JSON serialization. This is done for a few reasons:

• Keeps the incremental PR relatively small and easy to understand.
• Quickly gets us to a state where any new APIs added will automatically use Pigeon, reducing further accumulation of technical debt.
• Avoids duplication of handling code until [pigeon] Make the codec public flutter#150631 is resolved. As noted in a TODO added in this PR, almost all of the data structures that are passed in these methods are also passed through the PlatformView factory constructor, and Pigeon doesn't yet support using the Pigeon codec in non-Pigeon-generated code, so we would need to have both the structured and JSON handler for all of these objects if we converted them now.

Future PRs will be able to incrementally convert each map object to a structured form. Converting the Flutter APIs (Java->Dart) will also be done in a follow-up, to limit the scope of this PR.

This is the iOS equivalent of #6980, and the Dart code is largely the same.

Part of flutter/flutter#117907

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• 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 updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[jmagman]

nit

Suggested change

	- (void)testcameraUpdateFromArrayNewCameraPosition {
	- (void)testCameraUpdateFromArrayNewCameraPosition {

[jmagman]

Is the return string needed any more since you added the styleError setter side-effect?

[stuartmorgan]

Technically no, but I think it's easier to understand return [self.controller setMapStyle:style]; in the Pigeon method implementation than:

[self.controller setMapStyle:style];
// This value has to be read after the call above because it's set as a side effect of that call.
return self.styleError;