Adds 'IN' metadata filter for pgvector for checking set presence

[eyurtsev]

Adds "IN" metadata filter for pgvector to all checking for set presence

PGVector currently supports metadata filters of the form:

{"filter": {"key": "value"}}

which will return documents where the "key" metadata field is equal to "value".

This PR adds support for metadata filters of the form:

{"filter": {"key": { "IN" : ["list", "of", "values"]}}}

Other vector stores support this via an "$in" syntax. I chose to use "IN" to match postgres' syntax, though happy to switch.
Tested locally with PGVector and ChatVectorDBChain.

@dev2049

[dev2049]

thanks @jadespanning!