Migrate to Federated plugin structure

[renefloor]

This PR migrates the plugin to a federated plugin setup.
This allows devs to use a different type of platform map, for example if they don't like Google maps on Android but want to use something else, but more importantly, this removes the dependency on GoogleMaps from the iOS app.

Still to do

• Update Readme's and package descriptions
• Check if we can verify version numbers in the github workflow
• Update changelogs
• Update licenses

Issue #65

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.

[mvolpato]

It looks good to me. I run the example on iOS and it works well.