Allow multiple LOAD arguments with wildcard LOAD aggregations

[uglide]

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Description of change

Users should be able to use @__key attribute alongside with LOAD *. Currently, it's not possible to build an aggregation request for the command like this:

    res = r.execute_command(
        'ft.aggregate', 'games', '*', 'LOAD', '2', '@nonexist', '@price',
        'SORTBY', 1, '@nonexist', 'MAX', 10,
        'LOAD', '3', '@__key', 'AS', 'key',
    )