Support Multiple Hashing Function

[amishra-u]

Buildfarm supports multiple hash functions, but it requires the hashing function to be configured at service startup. This means that even though Buildfarm can support multiple hash functions, only one can be active at a time.

Since Bazel passes the hashing algorithm in the ByteStream:Write request and ExecuteService:Execute
API, Buildfarm can select the hashing algorithm per API request instead of configuring it during service startup.

[werkt]

This is now available as of #1870