[google_maps_flutter] Send internal usage attribution Id

### Use case

It was agreed with the Google team that `google_maps_flutter` should send platform-specific internal usage attribution Ids.

Attribution Ids must match the following regex: `(gmp_[a-z0-9]+_[a-z]+_v[\.0-9]+(?:_[a-z]+)?)")` and their length must be less than 50 characters.

Each Id should include platform name and current package version code: `gmp_flutter_googlemapsflutter_v_`

### Proposal

* Platform implementation sends attribution Id directly without exposing any APIs to user-facing plugin
* A CI job is added to check that native implementation's version matches the attribution Id version code. It'll be developer's responsibility to make sure both are correct with a CI check ensuring that

`google_maps_flutter_web` needs https://github.com/a14n/dart-google-maps/issues/144 to be implemented