Sqlite: add transaction savepoint API

[roji]

Sqlite supports the SQL Standard for savepoints: https://www.sqlite.org/lang_savepoint.html. SqlClient and Npgsql expose APIs for this their DbTransaction implementation, and dotnet/runtime#33397 tracks adding the relevant APIs in System.Data.

[bricelam]

System.Data.SQLite leveraged savepoints to implement nested transactions at the ADO.NET layer. I like the proposed design better.

[ajcvickers]

@roji @bricelam What is this blocked on? Should it be in 5.0 if it is blocked?

[bricelam]

I think it was blocked on getting it into ADO.NET. I don't think it needs to be in 5.0.

[roji]

@ajcvickers this is simply waiting for the new methods to appear in a new version of the SDK (I think preview8). I know it's technically an enhancement, but I think we should just do it. It's trivial to do, there's virtually no risk (new APIs) and it would make sense to implement the new 5.0 API in 5.0...

[ajcvickers]

If it can go in this week, okay. Otherwise we'll discuss on Friday.

[smitpatel]

@roji - Can you verify milestone/labels?

[roji]

Thanks for taking care of it @smitpatel