Adds support for setting the map appearance for light and dark mode manually.

[timbrueckner]

Introduces a new parameter appearanceMode which can be used to override the default color scheme of the apple map (dark/light).

Fixes: fluttercommunity#77, fluttercommunity#42

I wasn't able to get the Flutter Test Driver running. Maybe the project should migrate to the new integration tests.

Pre-launch Checklist

• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making if a test is possible.
• All existing and new tests are passing.