.NET 5.0 | Add async System.Data resultset and database schema APIs

[roji]

dotnet/runtime#38028 adds async schema APIs to System.Data, on both DbConnection and DbDataReader; these automatically fall back to calling the sync versions if not overridden. However, SqlClient will now be able to override them, and provide truly async support for these operations.

Note: this would require cross-targeting to net5.0.

[cheenamalhotra]

Thanks @roji

Acknowledged for .NET 5.0 new APIs.