Make it possible to have async methods that don't open a new scope for AsyncLocals

[erik-kallen]

Sometimes it's useful to have async methods that don't open a new scope for eg. AsyncLocals.

For example:

class AmbientTransaction
{
    public AsyncLocal<IDbTransaction> AmbientTransaction { get; } = new();

    public IDbTransaction BeginAndSetAmbientTransaction(IDbConnection connection)
    {
        var tx = connection.BeginTransaction();
        AmbientTransaction.Value = tx;
        return tx;
    }

    // This method does NOT work
    public async Task<IDbTransaction> BeginAndSetAmbientTransactionAsync(IDbConnection connection)
    {
        var tx = await connection.BeginTransactionAsync();
        AmbientTransaction.Value = tx;
        return tx;
    }
}

This is apparently possible with an awful lot of ugly reflection, but it would be nice to have some supported solution. I assume that the AsyncTaskMethodBuilderAttribute could be used (eg. [AsyncTaskMethodBuilder(typeof(UseParentExecutionContextMethodBuilder))].

I assume this could theoretically be implemented in user code, but it's rather complex and not much of the existing infrastructure can be reused because everything is internal.