.Net: Add PostgresVectorStore Memory connector.

[lossyrob]

This PR adds a PostgresVectorStore and related classes to Microsoft.SemanticKernel.Connectors.Postgres.

Motivation and Context

As part of the move to having memory connectors implement the new Microsoft.Extensions.VectorData.IVectorStore architecture (see https://github.com/microsoft/semantic-kernel/blob/main/docs/decisions/0050-updated-vector-store-design.md), each memory connector needs to be updated with the new architecture. This PR tackles updating the existing Microsoft.SemanticKernel.Connectors.Postgres package to include this implementation. This will supercede the PostgresMemoryStore implementation.

Some high level comments about design:

• PostgresVectorStore and PostgresVectorStoreRecordCollection get injected with an IPostgresVectorStoreDbClient. This abstracts the database communication and allows for unit tests to mock database interactions.
• The PostgresVectorStoreDbClient gets passed in a NpgsqlDataSource from the user, which is used to manage connections to the database. The responsibility of connection pool lifecycle management is on the user.
• The IPostgresVectorStoreDbClient is designed to accept and produce the storage model, which in this case is a Dictionary<string, object?> . This is the intermediate type that is mapped to by the IVectorStoreRecordMapper.
• The PostgresVectorStoreDbClient also takes a IPostgresVectorStoreCollectionSqlBuilder, which generates SQL command information for interacting with the database. This abstracts the SQL queries related to each task, and allows for future expansion. This is particularly targeted at creating a AzureDBForPostgre vector store that will enable alternate vector implementations like DiskANN, while leveraging the same database client as the Postgres connector.
•  The integration tests for the vector store utilize Docker.Net to bring up a pgvector/pgvector docker container, which test are run against.

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 😄

[lossyrob]

@westey-m most recent comments addressed, thanks!

[Hanake0]

Hi @westey-m and @lossyrob,

Thanks for the work on this feature.
Sorry for the inconvenience if that is not the right place to ask, but do you guys know when will it be available in the Microsoft.SemanticKernel.Connectors.Postgres package?

We are currently using the legacy Memory Store but that was recently marked as legacy and the connector for Postgres is currently marked as in-development, but as i see here it is probably at-least ready for alpha testing.

Is it available for testing already in any channel or is any ETA available for it?

Again, sorry if that is not the right place to ask.

Thanks!

[westey-m]

@Hanake0, no problem. It's been a bit busy lately with various deadlines, but I was meaning to get back to this again this week. With the start of the holiday period, giving an ETA will be difficult, but we'll certainly be progressing it as a priority.

[dmytrostruk]

A couple of small comments, but in general looks good to me! Thanks for your contribution!

[lossyrob]

@dmytrostruk @westey-m Thanks for getting this over the finish line! I was out on parental leave but am back and can take on any follow up work. Feel free to tag me in any related issues etc.