perf(fields): create index for instance domain query (#9146)

# Which Problems Are Solved

get instance by domain cannot provide an instance id because it is not
known at that time. This causes a full table scan on the fields table
because current indexes always include the `instance_id` column.

# How the Problems Are Solved

Added a specific index for this query.

# Additional Context

If a system has many fields and there is no cache hit for the given
domain this query can heaviuly influence database CPU usage, the newly
added resolves this problem.

cmd/setup/43/cockroach/43.sql

@@ -0,0 +1,3 @@
+CREATE INDEX CONCURRENTLY IF NOT EXISTS fields_instance_domains_idx
+ON eventstore.fields (object_id)
+WHERE object_type = 'instance_domain' AND field_name = 'domain';

cmd/setup/43/postgres/43.sql

@@ -0,0 +1,3 @@
+CREATE INDEX CONCURRENTLY IF NOT EXISTS fields_instance_domains_idx
+ON eventstore.fields (object_id) INCLUDE (instance_id)
+WHERE object_type = 'instance_domain' AND field_name = 'domain';