Support lists in external reference resolvers

[luke-pp]

Add support for the Apollo [External] attribute/directive on list types.

• Currently the ArgumentParser class does not support list types, meaning the reference resolver always receives null for the external list type even when a list is provided to it (see #7991 for details)
• This PR adds a test to the existing suite for the @external support, and an implementation for the ArgumentParser that lets it parse lists.
• The implementation is based on the existing implementation for the InputParser, although the function signature is a bit different
• There are a couple of edge cases that still need implementation, I'll need a bit of guidance to make sure they work as intended

Closes #7991

[CLAassistant]

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