Generalize interceptors and add connection and transaction interception

[ajcvickers]

Registration of interceptors is now command and moved to core:

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder
        .UseSqlite("DataSource=Test.db")
        .AddInterceptors(new MyRelationalInterceptor(), new MyLoggingInterceptor());
}

Creating chains (essentially multi-dispatch) is handled automatically, with a mechanism for new interceptors to integrate with the functionality.

Also, added a lot of async paths that will be needed for new ADO.NET async methods. Part of #15917