[google_maps_flutter] Allow (de-)serialization of CameraPosition

[duzenko]

Description

Make CameraPosition.toMap public and change visibility of CameraPosition fromMap.
Intended use:

              final s = json.encode(lastPosition.toMap());
              prefs.setString('cameraPosition', s);
...
              final map = json.decode(prefs.get('cameraPosition'));
              final cam = CameraPosition.fromMap(map);
              await controller.moveCamera(CameraUpdate.newCameraPosition(cam));

Related Issues

Fixes flutter/flutter#34321

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
  Presumably none needed
• All existing and new tests are passing.
  Impossible: [flutter_plugin_tools] 'format' crashes on Windows flutter#33964
• I updated/added relevant documentation (doc comments with ///).
  None found
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• [] I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
  Pending review
• I updated CHANGELOG.md to add a description of the change.
  Pending review
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[iskakaushik]

Change looks good. Can you update the pubspec and changelog? Thanks!

[iskakaushik]

@duzenko I was busy with some issues in the past couple of weeks. Let me know if you get a change to fix the merge conflicts and we can merge this.

[iskakaushik]

Now that marker drag events is in. Can you please rebase?

[iskakaushik]

@duzenko I'm trying to figure out what is wrong with the plugin tools format to drive #1821 to green and get it merged. For the other PR it will get prioritized per the number of thumbs-up in the issues.