Async resultset and database schema APIs

[bgrainger]

API proposal at dotnet/runtime#38028.

Note that GetSchemaTableAsync and GetColumnSchemaAsync will always run synchronously in MySqlConnector; this should be called out in the documentation.

[bgrainger]

Updated API proposal at dotnet/runtime#38028 (comment).

[roji]

@bgrainger I've just merged dotnet/runtime#39098 for async schema APIs. If your implementation is sync in any case, there's no real reason to do anything on your side - the new System.Data APIs automatically fall back to their sync counterparts. Though I'm curious why these can only be synchronous on MySQL, is there no network involved?

[bgrainger]

@roji When a query is executed, MySQL Server sends the column metadata (for each column in the result set) as the initial response, so this information is always available by the time GetColumnSchemaAsync is invoked.

[roji]

@bgrainger what about DbConnection.GetSchemaTable, which is supposed to provide the list of tables/columns/views/etc?

For the DbDataReader, the standard seems to be to expose the minimal information that's provided by the server, but if CommandBehavior.KeyInfo is specified on the command, to possibly go back to the database and retrieve any extra information via another query. No idea whether that makes sense for MySQL or not...

[roji]

At least for Npgsql, PostgreSQL doesn't send a lot of information with standard resultsets, so I use KeyInfo is an indicator that the user wants me to query additional info from the database. I can provide more details (the relevant file is DbColumnSchemaGenerator).