Propose vector-set API

[mgravell]

as per https://redis.io/docs/latest/develop/data-types/vector-sets/

• all methods/types start VectorSet...
• all core methods implemented
• the only "unusual" one is VLINKS; the server returns this as nested data, but the nesting is not meaningful to the caller (instead being related to the server core), so I've flattened the result
• usage is shown in VectorSetIntegrationTests, in particular VectorSetSimilaritySearch_WithFilter is useful for overview - since the main two primary APIs are VADD and VSIM
• since vector set data can be non-trivial, all return types lean on Lease<T> rather than T[]. Inputs are ReadOnlyMemory<T>; the exception to this is string data for JSON and filters; these are explicitly text, never blobs, so RedisValue seems inappropriate. I think forcing string is OK here
• in acknowledgement that some of our files are too large, I've started splitting the VectorSet* bits out via partial files; I will follow this up with "everything else" (.Strings.cs, .Hashes.cs) in a separate PR

This PR also introduces the start of a new literal-matching API, ala FastHash. It is intended that this will be extended at a later date.

CI: may be dependent on the vectorset module; if it fails, I'll add suitable validation. All VectorSetTests pass locally:

[mgravell]

Additional thoughts on FastHash:

• if length <= 8, we can skip equality test
• consider hashing first 16 (status: considered and deferred)
• add unit test that shows interesting known values of different lengths (and different values at same length)