perf(cache): pgx pool connector (#8703)

# Which Problems Are Solved

Cache implementation using a PGX connection pool.

# How the Problems Are Solved

Defines a new schema `cache` in the zitadel database.
A table for string keys and a table for objects is defined.
For postgreSQL, tables are unlogged and partitioned by cache name for
performance.

Cockroach does not have unlogged tables and partitioning is an
enterprise feature that uses alternative syntax combined with sharding.
Regular tables are used here.

# Additional Changes

- `postgres.Config` can return a pxg pool. See following discussion

# Additional Context

- Part of https://github.com/zitadel/zitadel/issues/8648
- Closes https://github.com/zitadel/zitadel/issues/8647

---------

Co-authored-by: Silvan <silvan.reusser@gmail.com>

internal/cache/pg/prune.sql

@@ -0,0 +1,18 @@
+delete from cache.objects o
+where o.cache_name = $1
+	and (
+		case when $2::interval > '0s'
+			then created_at < now()-$2::interval -- max age
+			else false
+		end
+		or case when $3::interval > '0s'
+			then last_used_at < now()-$3::interval -- last use
+			else false
+		end
+		or o.id not in (
+			select object_id
+			from cache.string_keys
+			where cache_name = $1
+		)
+	)
+;