[Task]: [RRIO] [Cache] Redis based DoFn supporting reads

[damondouglas]

What needs to happen?

The output of this task is a Redis based DoFn that supports cache reads. Implementation re-uses the Call PTransform and therefore is a dependency.

Measures of Ready

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

Measures of Done

• class CacheReadRedisFn<RequestT, ResponseT> extends DoFn<RequestT, KV<RequestT, ResponseT>>
• Tests validate that DoFn emits existing RequestT, ResponseT pairs, null otherwise
• Tests validate decoded elements are not corrupted

Issue Priority

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