xds: explicitly set request hash key for the ring hash LB policy

[danielzhaotongliu]

Implements gRFC A76: explicitly setting the request hash key for the ring hash LB policy

• Explictly setting the request hash key is guarded by the GRPC_EXPERIMENTAL_RING_HASH_SET_REQUEST_HASH_KEY environment variable until API stabilized.

Tested:

• Verified end-to-end by spinning up multiple gRPC servers and a gRPC client that injects a custom service (load balancing) config with ring_hash_experimental and a custom request_hash_header (with NO associated value in the metadata headers) which generates a random hash for each request to the ring hash LB. Verified picks/RPCs are split evenly/uniformly across all backends.
• Ran affected unit tests with thread sanitizer and 1000 iterations to prevent data races.

CC: @shivaspeaks