Mark required parameters throughout API with @required

[bkonyi]

In some situations it's unclear which parameters are required, particularly for the OAuth flow. We should investigate whether or not there would be any benefit of marking parameters as @required, although this may be handled after non-nullability lands in Dart.

[misterfourtytwo]

i guess it should be closed now after migration to null-safety, where required is kinda mandatory now.
or i could do this one here on coming weekend, if this task is still necessary.