Re-implementing the Postgres memory store

[JadynWong]

Motivation and Context

Reimplement Postgres in-memory storage based on the discussion in #1338.

Description

• Re-implementing the Postgres memory store, mapping between SK collection to Postgres table.
• PostgresMemoryStore no longer implements IDisposable pattern.
• No longer execute the enable pgvector extension statement, add it to README.md to be executed by the user. It only needs to be executed once in the database, and the extension may be enabled differently for different hosting methods.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[JadynWong]

Hi @dmytrostruk,
Following the previous discussion on implementing the new design, I hope you can review it and give suggestions.
Thank you.

[dmytrostruk]

@JadynWong thank you for contribution and quick action for Postgres memory store. While this implementation has better design and resolves a lot of issues of previous version, there are still a couple of problems to resolve. I think they are not critical but will improve the functionality even more. Thanks again!

[dmytrostruk]

@JadynWong thank you for quick action on PR comments! I tested the functionality using integration tests together with Example39_Postgres - it works as expected! 🎉
Thanks for adding unit tests, taking into account that integration tests are temporarily ignored, unit tests will increase our code coverage.

I left minor comments in documentation and tests. As soon as they are resolved, this PR will be ready to merge to main.

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Ideas for improvements in future (this can be done in follow-up PRs):
Since we have PostgresDbClient which is responsible for sending Postgres queries, it would be better if we operate in this class with Postgres entities.

For example, IPostgresDbClient interface has method GetCollectionsAsync, but there is no entity collection in Postgres, there is table. So, what if methods in IPostgresDbClient will be like GetTablesAsync, CreateTableAsync etc?

Another example, in PostgresDbClient we have method CreateCollectionAsync and in this method there are two calls CreateTableAsync and CreateIndexAsync, which means that this class defines a logic for creating specific table in Postgres which should act as SK collection, but this is responsibility of PostgresMemoryStore class.

If we follow single responsibility principle, there would be:

• PostgresDbClient - which operates only with Postgres entities and is responsible for sending simple queries for create/delete tables, records, search.
• PostgresMemoryStore - which will work as adapter between Semantic Kernel and Postgres, using PostgresDbClient to operate with Postgres entities.

Same applies to other connectors where we have the same problem.

[JadynWong]

Ideas for improvements in future (this can be done in follow-up PRs): Since we have PostgresDbClient which is responsible for sending Postgres queries, it would be better if we operate in this class with Postgres entities.

For example, IPostgresDbClient interface has method GetCollectionsAsync, but there is no entity collection in Postgres, there is table. So, what if methods in IPostgresDbClient will be like GetTablesAsync, CreateTableAsync etc?

Another example, in PostgresDbClient we have method CreateCollectionAsync and in this method there are two calls CreateTableAsync and CreateIndexAsync, which means that this class defines a logic for creating specific table in Postgres which should act as SK collection, but this is responsibility of PostgresMemoryStore class.

If we follow single responsibility principle, there would be:

• PostgresDbClient - which operates only with Postgres entities and is responsible for sending simple queries for create/delete tables, records, search.
• PostgresMemoryStore - which will work as adapter between Semantic Kernel and Postgres, using PostgresDbClient to operate with Postgres entities.

Same applies to other connectors where we have the same problem.

Great advice, I'd like to finish it in a follow up PR.

[dmytrostruk]

LGTM!