perf(eventstore): add instance position index (#9837)

# Which Problems Are Solved

Some projection queries took a long time to run. It seems that 1 or more
queries couldn't make proper use of the `es_projection` index. This
might be because of a specific complexity aggregate_type and event_type
arguments, making the index unfeasible for postgres.

# How the Problems Are Solved

Following the index recommendation, add and index that covers just
instance_id and position.

# Additional Changes

- none

# Additional Context

- Related to https://github.com/zitadel/zitadel/issues/9832

cmd/setup/54.sql

@@ -0,0 +1 @@
+CREATE INDEX CONCURRENTLY IF NOT EXISTS es_instance_position ON eventstore.events2 (instance_id, position);