Use DB dumps for K8up backups

[ccremer]

Summary

• Switches K8up backups from simple PVC file backup to DB dumps using pg_dump
• Adds a how-to page that guides through a DB data restoration process
• Follows #69

Checklist

For Code changes

• [x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
• [x] PR contains the label area:operator
• [x] Link this PR to related issues
• [x] I have not made any changes in the charts/ directory.

[ccremer]

For review: If you want to verify that restoration works, follow roughly this procedure:

1. make clean -> ensure that there's no existing backups in Minio
2. makelocal-install install-samples s3-credentials
3. Patch the generated K8up Schedule so it does a backup every 3 minutes (*/3 * * * *) (e.g. with kubectl edit)
4. Wait until a backup succeeds
5. Follow the how-to until/including step 4 (just before restoring data)
6. Insert some fake data:

   $ kubectl exec -i -t -n sv-postgresql-s-evident-yellowjacket-f9f2 postgresql-0 -c postgresql -- sh -c "clear; (bash || ash || sh)"
   $ (inside container) PGPASSWORD=$POSTGRES_PASSWORD psql -U $POSTGRES_USER
   $ (inside psql) CREATE TABLE phonebook(phone VARCHAR(32), firstname VARCHAR(32), lastname VARCHAR(32), address VARCHAR(64));
   $ (inside psql) INSERT INTO phonebook(phone, firstname, lastname, address) VALUES('+1 123 456 7890', 'John', 'Doe', 'North America');
   $ (inside psql) SELECT * FROM phonebook ORDER BY lastname;
         phone      | firstname | lastname |    address    
   -----------------+-----------+----------+---------------
    +1 123 456 7890 | John      | Doe      | North America

7. Now, wait until backup happens again using the newly-added data
8. Select the newest snapshot ID, and verify that the dump contains your data: restic dump --quiet ${SNAPSHOT_ID} /${deployment_namespace}-postgresql.sql
9. Insert more data:

   $ (inside psql) INSERT INTO phonebook(phone, firstname, lastname, address) VALUES('+1 123 456 7890', 'Hans', 'Muster', 'Europe');
   $ (inside psql) SELECT * FROM phonebook ORDER BY lastname;
         phone      | firstname | lastname |    address    
   -----------------+-----------+----------+---------------
    +1 123 456 7890 | John      | Doe      | North America
    +1 123 456 7890 | Hans      | Muster   | Europe

10. Restore the backup from dump (step 5). This should remove the line with "Hans Muster"
11. Verify that there's only the data from the backup:

    $ (inside psql) SELECT * FROM phonebook ORDER BY lastname;
          phone      | firstname | lastname |    address    
    -----------------+-----------+----------+---------------
     +1 123 456 7890 | John      | Doe      | North America

[ccremer]

Converting back to draft. I noticed that as of now, only the primary database (as defined by $POSTGRES_DB) is going to be backed up (or restored). I'll first investigate a bit into possibilities that all DBs are going to be backed up and restored.

[ccremer]

I was able to come up with a solution that is able to backup and restore all databases. This PR is now reviewable again.