Let CitusDB invalidate pg_shard's shard interval cache

`pg_shard` keeps a cache of shard interval metadata within each session which is _never_ refreshed. Within `pg_shard` this is fine, since all shards are created up front: caching forever isn't wrong. But now that Citus integration is more full-fledged, we're noticing that things like appending new shards or rebalancing existing ones results in `pg_shard` operating using stale metadata.

We need to come up with some sort of contract to let other pieces of software tell `pg_shard` about changes in the shard intervals of a distributed table. We may be able to reuse some existing pieces of invalidation logic within PostgreSQL, but that requires some reading to determine.