[google_maps_flutter] Migrate platform interface to null safety (#3539)

Part of flutter/flutter#75236

Includes a significant refactor of the various overlay objects to remove a lot of identical code across the objects.

Author: stuartmorgan-g

packages/google_maps_flutter/google_maps_flutter_platform_interface/CHANGELOG.md

@@ -1,3 +1,12 @@
+## 2.0.0-nullsafety
+
+* Migrated to null-safety.
+* BREAKING CHANGE: Removed deprecated APIs.
+* BREAKING CHANGE: Many sets in APIs that used to treat null and empty set as
+  equivalent now require passing an empty set.
+* BREAKING CHANGE: toJson now always returns an `Object`; the details of the
+  object type and structure should be treated as an implementation detail.
+
 ## 1.2.0
 
 * Add TileOverlay support.