cleosrv: Add 'backup' sub-command

To make it easy for users to back up their database.

`cleosrv backup --output ./my-backups/`

Include the currently used database and config file. ZIP file with the current datetime in the name. `cleosrv-backup-2022-01-03T13.00.00.zip` (colons in file names can be a problem) or something like that.

The `--output` parameter could be optional, in which case it could back up to .local/share/cleosrv/backups/ .

Start by locating the cleosrv command entry point and the existing handling for the active database and config file. Done means a backup sub-command creates a ZIP containing both, uses a datetime-based filename without problematic colons, and supports the specified output directory or the proposed default backup location.

Written by the indexing model from the issue text.