[Task]: [RRIO] [Call] PTransform executes user custom code without adaptive throttling

[damondouglas]

What needs to happen?

The output of this task is an implementation of the a Call PTransform and related DoFn that executes custom user code as defined in #28710. The DoFn adheres to the following specification:

class CallFn<KeyT, RequestT, ResponseT, ApiErrorT extends ApiException> 
    extends DoFn<KV<KeyT, RequestT>, KV<KeyT, ResponseT>> {}

Measures of Ready

• [Task]: [RRIO] [Test] Create test Caller and SetupTeardown interface implementations #28710

Measures of Done

• class Call<RequesT, ResponseT> extends PTransform<PCollection<RequestT>, Call.Result>
• class Result<RequestT, ResponseT> implements POutput
• class CallFn<RequestT, ResponseT> extends DoFn<RequestT, ResponseT>
• CallFn executes custom code during setup, teardown and process lifecycle methods
• Tests validate error messages expected from invalid parameter inputs
• Integration tests validate calls to deployed endpoint

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)