Consider caching `IntrospectionQuery` results

See this discussion: error-budget-reports#22 (comment 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.