`as` parameter

[rbalicki2]

• add an as parameter to MagicMutationFieldInfo, which should have type SelectableFieldName (if that deserializes with no issue), or otherwise StringLiteralValue (it will be converted to a SelectableFieldName later)
• The field name (e.g. https://github.com/isographlabs/isograph/blob/c24240805b20df5aa319a9c21c3bebd1aa0b4975/crates/isograph_schema/src/magic_mutation_fields.rs#L218) should be value of the exposeAs parameter, otherwise default to its existing value
• We should modify one of the @exposeField directives to use the new as parameter

(Note: I've called this exposeAs, but as seems better. But open to feedback on the naming choice.)