Mockable plugin instances & methods

[Ehesp]

Bug report

Currently to access the reworked plugins, you access them via statics, e.g:

FirebaseFirestore.instance

Since they're statics, they are not mockable.

Solutions

One solution proposed by @rrousselGit is to expose a mock static method for each plugin, e.g:

void main() {
  final mock = FirebaseFirestore.mock();

  afterEach(mock.reset);

  test('foo', () {
    mock.mockDocument('key', {'foo': 42});
  });
}

[TahaTesser]

Just adding the core label to get this issue out of backlog since this is for all packages, not sure we need to add all the package labels