Initial poc of initdb job

[eberlep]

Problem

From time to time, the need for custom SQL statements arises, but I haven't found a baked in way in the Spilo images to do so.

Proposed solution

A possible (and also image-independant) solution is a simple job: Created at the same time as the database, it restarts until it ran successfully. This job simply runs psql, connects to the master pod and executes the configured sql statements:

 ~  $ kgp
NAME                                 READY   STATUS      RESTARTS   AGE   SPILO-ROLE
pgfits-philippe1696f61449bc-0        3/3     Running     0          44s   master
postgres-initdb-5bwwn                0/1     Error       0          24s   
postgres-initdb-r2rr5                0/1     Completed   0          4s    
postgres-initdb-wsxbv                0/1     Error       0          51s   
postgres-initdb-x6jlh                0/1     Error       0          34s   
postgres-operator-67565dcbc6-fsbj4   1/1     Running     0          51s

TODOs

• Migration: Should existing DBs also execute that statement? YES
• Reconcile: Should updated statements be executed again? NO
• Statements configurable per DB or per Partition? PER PARTITION
• Move Statements to central ConfigMap in postgreslet-system namespace, similar to the Pod Environment Configmap
• Pass along the Spilo image to use
• Set ttlSecondsAfterFinished on job (K8s >= 1.23)?
• Don't execute on clones / standbies

[eberlep]

Finish up and merge

[eberlep]

Reviewed with Tom.