Implement MySqlConnection.CloseAsync

[bgrainger]

Connector/NET exposes a MySqlConnection.CloseAsync API (albeit implemented synchronously).

We could add the same API for clients who want to close the connection asynchronously; however note that in most cases this is just returning the connection to the pool, which is a synchronous operation (so CloseAsync would just add overhead).

[sazarubin]

Since it isn't an asynchronous operation, is not part of ADO.NET DbConnection's interface, and is needed only for backwards compatibility with Connector/NET, I think CloseAsync is better to be implemented as an extension, maybe even in a separate package.

[bgrainger]

(so CloseAsync would just add overhead).

This can mostly be avoided by not implementing it as an async method and instead just returning Task.CompletedTask.

[bgrainger]

Added in 0.57.0.