.Net: Add vectorless search to record collection interface.

[westey-m]

Motivation and Context

Many of the databases that implement the vector store abstractions support search without vectors.
It is possible to search the database with a zero value vector anyway, but doing vector comparisons
without needing to is inefficient.

Description

• Adding a vectorless search method to the collection interface with implementations.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[dmytrostruk]

I'm not sure this method should be a part of IVectorStoreRecordCollection. For me, it looks like vectorless search capability should be a part of either:

1. Separate interface, for those vector databases which can support that. Based on the XML doc since not all vector stores support searching without a vector, we may end up with some workaround implementations or throwing NotSupported/NotImplemented exception, which will be a bad pattern. So, I would avoid forcing developers to implement it.
2. This interface, but inside GetAsync method, which we can re-think. Currently, it accepts a key parameter, which is required to get a record, but we can re-design it to accept a filter instead of key parameter, and filter in turn can have a key or any other property in order to get a record. Then each connector will process the filter only in the way it natively supports (e.g. If getting a record by other property than key is not supported - throw NotSupported exception). Then, in order to simplify things for getting records by key, we can create an extension method which will accept a key parameter, create a filter by key and use GetAsync interface method. This will also allow to avoid breaking change.