Allow to temporarily disable WAL archiving

[eberlep]

To reduce the storage size and time needed when importing large amounts of data, especially during database migratinos, it should be possible to temporarily disable the archiving of wals.

A similar thing was proposed to the operator here, but was ultimately supercede by the generic approach of defining pod environment variables within the custom resource.

So we could introduce a new field to our CR called disableWALG, which would in turn cause the postgreslet to override the WALG_S3_BUCKET Environment Variable in the postgresql.acid.zalan.do custom resource.

Additionally, it was proposed to also set numberOfInstances to 1 and set synchronous_commit to false.