add support for inlining `Command` inside another `Command`

[stereobutter]

Some commands in the redis ai module accept other (redis ai) commands as arguments. It should be possible to inline commands similar to how Args works

Example:

sub_cmd = Command('AI.TENSORSET {tensor} FLOAT {shape} VALUES {values}', 
    tensor='foo', 
    shape=Args([2, 2]), 
    values=Args([1, 2, 3, 4]))
    
Command('AI.DAGEXECUTE ... |> {}', sub_cmd)