Consider caching `IntrospectionQuery` results

See this discussion: https://gitlab.com/gitlab-org/error-budget-reports/-/issues/22#note_1368382223

Querying on queries > 5s shows that ~68% of the queries are from getAllProjects, ~18% from IntrospectionQuery, ~13% from Missing, and 0.10% from other queries.

It seems that this query is requested a lot and it is slow.
It also does not change that often - only when we update the schema.
So it's not unique per user, only for the given schema snapshot at this moment of time.

It could be a good candidate for caching.