cubicweb #1500820 add a cubicweb-ctl command to handle CWSources via files [open]

I need a command to get the configuration for a CWSource as a file that I can edit / backup, and a command to upload a modified file to a CWSource config.

This is a must have when backups are restored from an environment to another one, where the configuration of the LDAP server is different.

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