fix(fields): add index to improve search by aggregate (#8267)

# Which Problems Are Solved

During performance testing of the `eventstore.fields` table we found
some long running queries which searched for the aggregate id.

# How the Problems Are Solved

A new index was added to the `eventstore.fields`-table called
`f_aggregate_object_type_idx`.

# Additional Changes

None

# Additional Context

- Table was added in https://github.com/zitadel/zitadel/pull/8191
- Part of https://github.com/zitadel/zitadel/issues/7639

cmd/setup/31.sql

@@ -0,0 +1 @@
+CREATE INDEX CONCURRENTLY IF NOT EXISTS f_aggregate_object_type_idx ON eventstore.fields (aggregate_type, aggregate_id, object_type);