cache: add redis key prefix support

[cnaples79]

Summary

• add optional field to Redis cache config
• prefix all redis keys in get/set operations when configured and return hits keyed by original keys
• add a prefixed-config test case to cover the new behavior

Rationale

• allows isolating cache keys when sharing a Redis instance or DB between components; defaults to empty string so existing setups are unaffected

Test Plan

• unit tests updated (redis client tests include prefixed config)
• all tests passing locally

Fixes #8608

[GiedriusS]

But it's the intention to share results between instances so that they could re-use the results of computation. If the problem is that the keys&values are taking too much space, certain clusters or subset of services can use a different Redis cluster. Could you please elaborate on what problem you're trying to solve?

(Added a comment on the original issue, let's continue the discussion there).

[GiedriusS]

Could we please add two separate paths - with and without a prefix? Otherwise, this very hot path will get 2 extra allocations even if one isn't using a prefix.

[GiedriusS]

@cnaples79 ping, just one nit and we can merge this.