Add standard backup labels/annotations to volume snapshots #2794

Once the volume snapshot through cold backup is taken, we should add some labels to the object to facilitate search via selectors, as well as some annotations containing the relevant information. This is just a basic initial idea.

Labels:

• cnpg.io/backupTimeline
• cnpg.io/backupYear: format YYYY
• cnpg.io/backupMonth: format YYYMM
• cnpg.io/onlineBackup: true/false - false in case of cold backup (meaning the backup is consistent)

Annotations:

• cnpg.io/backupStartWAL
• cnpg.io/backupEndWAL
• cnpg.io/backupStartTime
• cnpg.io/backupEndTime
• cnpg.io/snapshotStartTime - taken before invoking the creation of the snapshot
• cnpg.io/snapshotEndTime - taken when the volume snapshot becomes ready (approximation).

My suggestion is to do further analysis on pg_controldata and what we will expect from future support of backup labels returned by pg_start_backup() and pg_stop_backup(). More ideas can be taken from the backup info file in Barman.