Add pg-decomp dashboard based on pg-upgrade dashboard

This MR aims to add a decomposition dashboard based on the pg-upgrade dashboard. Since the pg-upgrade dashboard works on a single cluster with multiple major versions (i.e. "patroni-main-v14" and "patroni-main-v16") and decomposition needs to operate with two separate clusters on the same version (i.e. "patroni-main-v16" and "patroni-sec-v16"), there were changes required, hence the new dashboard.

Example dashboard

• Example dash: https://dashboards.gitlab.net/goto/rz6Uqd2NR?orgId=1
• Existing upgrade dash: https://dashboards.gitlab.net/d/postgresql-pg-upgrade/postgresql3a-postgres-upgrade-using-logical

Testing

Attempted to run ./test-dashboard.sh:

jonathon@jsisson--20240506-WT9MH dashboards % ./test-dashboard.sh postgresql/pg-decomp.dashboard.jsonnet 
/opt/homebrew/bin/op
API call to https://dashboards.gitlab.net/api/snapshots failed with 403:
{"message":"forbidden","traceID":""}

This was using the 1password data to add the test, so I don't have a snapshot preview.