Add support for argsfile inputs to BXL

The `buck2` CLI supports passing build targets via files, which is important when the list might be larger than the maximum CLI args length.
Given that BXL supports taking target patterns as inputs, it would be very convenient if it could support argfiles as well, given that there is (to my knowledge) no way to implement them by hand.

Would this be a welcome change? If so, could someone give me some pointers as to how to go about it?

I'm also open to any suggestions for how to solve my problem (passing a large amount of build targets to a BXL script) some other way.