Provide a way to map a given value to its corresponding shard in Vitess

[rohit-nayak-ps]

Feature Description

At times we need to know which shard a specific vindex value will be mapped for a given cluster configuration.
Essentially a tool or RPC that gives the mapped shard given

• the vindex type (e.g., hash, xxhash) and
• shard range

Use Case(s)

• For debugging and testing to ensure that a specific row is mapped to the correct shard
• For multi-tenant or geo-sharded clusters we may want to know the shard for specific tenants/geo regions to locate them in specific cloud regions
• We may want to provision certain shards differently because they house enterprise customers