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>

cmd/setup/34/cockroach/34_cache_schema.sql

@@ -0,0 +1,27 @@
+create schema if not exists cache;
+
+create table if not exists cache.objects (
+	cache_name varchar not null,
+    id uuid not null default gen_random_uuid(),
+    created_at timestamptz not null default now(),
+	last_used_at timestamptz not null default now(),
+    payload jsonb not null,
+
+	primary key(cache_name, id)
+);
+
+create table if not exists cache.string_keys(
+    cache_name varchar not null check (cache_name <> ''),
+    index_id integer not null check (index_id > 0),
+    index_key varchar not null check (index_key <> ''),
+    object_id uuid not null,
+
+    primary key (cache_name, index_id, index_key),
+    constraint fk_object
+        foreign key(cache_name, object_id)
+        references cache.objects(cache_name, id)
+        on delete cascade
+);
+
+create index if not exists string_keys_object_id_idx
+    on cache.string_keys (cache_name, object_id); -- for delete cascade