Add encryption option to backup schedule

Created MR with added support for Encryption in scheduled backups.
Now when creating a schedule there is Backup Encryption fieldset which will be disabled if defuse php encrpytion is not installed and appropriate message is shown:

If the library is installed then you can enable/disable Encryption and set password:

Also in case of update user is asked if he wants to change the password, since pw field doesn't have default value and this will prevent overwritting password each time Schedule form is saved again.

There is also an update hook which makes sure that previous Schedule configs have default value for new encrypt fields.
Since this PR has update hook, it would be good to also merge this PR: https://www.drupal.org/project/backup_migrate/issues/3338192 🐛 backup_migrate_update_5001 is never executed due to CORE_MINIMUM_SCHEMA_VERSION Fixed
in order to get correct execution sequence.

Thanks for working on this, we'll take a look at it soon.

FYI as a reminder the "Assigned" field is for indicating that you're actively working on an issue, once you're finished please set the field to "Unassigned". Thank you.

Gotya about Assigned field.
I updated PR with new schema number for update hook (50002).

Tested this and works as expected, I can see the new "Encrypt" field in the "Schedule" config page, tested it as well, can be moved to RTBC if needed.

Did you test that the backup was encrypted?