Support the new savepoint API

[roji]

Support for savepoints has been merged into EF Core via dotnet/efcore#20176, this tracks adding support on the Pomelo provider side.

This is super simple to do - simply subclass from RelationalTransaction and implement the relevant methods, following what was done for Sqlite in dotnet/efcore#20710 (SQL Server is lacking support for Release).

[bgrainger]

The Savepoint API was added to MySqlConnector in mysql-net/MySqlConnector#775.

[roji]

Thanks @bgrainger.

Note that since I opened this issue, I improved the default relational support so that the Pomelo provider probably doesn't need to do anything - any provider using RelationalTransaction will automatically attempt to use the standard savepoint management SQL (SAVEPOINT x, ROLLBACK TO x, RELEASE SAVEPOINT x). IIRC MySQL supports this, so apart from possibly enabling testing this should just work.

[roji]

To expand on the above, the savepoint support in EF Core 5.0 does not rely on the new System.Data APIs being introduced to .NET 5.0 (dotnet/runtime#33397). In the future, if EF Core drops support for pre-.NET 5.0 (or cross-targets) we can have RelationalTransaction rely on the System.Data API instead of doing its own thing.

[lauxjpn]

I'll close this one, since EF Core does all the magic for us here.