Async System.Data resultset and database schema APIs

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Provider tracking issues

This new API has been merged, following are tracking issues for implementation in the different providers:

• SqlClient: .NET 5.0 | Add async System.Data resultset and database schema APIs  SqlClient#646
• Npgsql: Introduce async schema APIs on connection and reader npgsql/npgsql#2976
• MySqlConnector: Async resultset and database schema APIs mysql-net/MySqlConnector#835

Background and Motivation

This proposal introduces async counterparts to existing sync APIs for fetching schema information for resultset and database schema. The original proposal was developed by @manandre and @bgrainger in npgsql/npgsql#2976.

Proposed API

public class DbConnection
{
    public virtual Task<DataTable> GetSchemaAsync(string collectionName = null, string[] restrictions = null, CancellationToken cancellationToken = default);
}

public class DbDataReader
{
    public virtual Task<DataTable> GetSchemaTableAsync(CancellationToken cancellationToken = default);
}

public interface IDbColumnSchemaGenerator
{
    Task<ReadOnlyCollection<DbColumn>> GetColumnSchemaAsync(CancellationToken cancellationToken = default);
}

public static class DbDataReaderExtensions
{
    public static Task<ReadOnlyCollection<DbColumn>> GetColumnSchemaAsync(this DbDataReader reader, CancellationToken cancellationToken = default);
}

Notes

• The default implementations of the above will call the appropriate sync counterpart, as is standard in ADO.NET.
• As these APIs aren't perf-sensitive, they return Task instead of ValueTask. Since they typically return a considerable amount of data, the extra allocation is unlikely to matter in any case. They also mostly return asynchronously.

/cc @David-Engel @cheenamalhotra @bgrainger @manandre @bricelam @ajcvickers @stephentoub @terrajobst @mgravell @FransBouma