Add key_prefix and db configuration options

[dinceraslancom]

Description

This update fixes issues in RedisStore and RedisScheduler from #1585.

Use key_prefix to add a custom prefix to all keys.
Choose a db number instead of always using 0.

Fixes # (1585)

Type of change

• [ x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Configured nativelink-config/examples/redis.json5 with both key_prefix: "my_test_prefix:" and db: 1.
Started Redis locally using Docker.
Started Nativelink using bazel run //:nativelink -- $(pwd)/nativelink-config/examples/redis.json5
Used redis-cli MONITOR to watch Redis commands.
See Redis commands always using keys with my_test_prefix:.

Checklist

• [ x ] bazel test //... passes locally
  This PR contains two distinct logical commits (one for key_prefix, one for db selection) for clarity in history

---

This change is 

[CLAassistant]

All committers have signed the CLA.

[MarcusSorealheis]

Thank you for the contribution. It's almost ready for review. Simple fix here:

 > index 38f06c8..5cd5b44 100644
       > --- a/nativelink-config/examples/redis.json5
       > +++ b/nativelink-config/examples/redis.json5
       > @@ -75,4 +75,4 @@
       >        }
       >      }
       >    ]
       > -}
       > \ No newline at end of file
       > +}

you only need an empty line at the end of /nativelink-config/examples/redis.json5.

Granted, I haven't tested the functionality, but I will. @barrbrain and @aaronmondal and @kubevalet will also be keen to test it.

[dinceraslancom]

@MarcusSorealheis Thank you for the feedback. I've fixed that part.

[MarcusSorealheis]

I've merged with main to catch up and observe if anything new breaks.

[MarcusSorealheis]

@dinceraslancom You need to sign the CLA?

[MarcusSorealheis]

Could you also add a test?

[dinceraslancom]

@MarcusSorealheis I already confirmed the CLA, but there was an issue with email matching, solved.

I will write test case.