[RRIO] [Call] Consider many requests to one API call use cases

[damondouglas]

This issue resulted in response to #29144 (comment). The output of this task is to explore ways to support API endpoints that involve batched requests. In this context, an API call has many requests to one API call and response cardinality.

Measures of Ready

• [Task]: [RRIO] [Call] PTransform executes user custom code without adaptive throttling #28711

Measures of Done

• Benchmarks show a measurable performance improvement with or without this task's implementation
• A Caller implementation accepts a list of requests
• Call PTransform's DoFn parallelizes Caller invocations

[github-actions]

This issue has been marked as stale due to 150 days of inactivity. It will be closed in 30 days if no further activity occurs. If you think that’s incorrect or this issue still needs to be addressed, please simply write any comment. If closed, you can reopen the issue at any time. Thank you for your contributions.