listSize directive parsing improvements

[morriswchris]

Improvements to @listSize argument parsing:

• Array parsing for @listSize: List-size directives can use array-style parsing for sizing (e.g. list arguments). This allows queries like itemsByUUID(ids: [ID!]) to be accurately costed
• Nested input paths: Nested input paths are supported when resolving list size from query arguments.
• Nested sizedFields: The sizedFields argument on @listSize supports nested field paths for more accurate cost estimation.

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[apollo-librarian]

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[aaronArinder]

rubberstamp for already-reviewed code (apart from changeset, which looks good)