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/set.sql

@@ -0,0 +1,19 @@
+with object as (
+	insert into cache.objects (cache_name, payload)
+	values ($1, $3)
+	returning id
+)
+insert into cache.string_keys (
+	cache_name,
+	index_id,
+	index_key,
+	object_id
+)
+select $1, keys.index_id, keys.index_key, id as object_id
+from object, jsonb_to_recordset($2) keys (
+	index_id bigint,
+	index_key text
+)
+on conflict (cache_name, index_id, index_key) do
+	update set object_id = EXCLUDED.object_id
+;