Add support for get-data

[GeoffAliro]

Add support for get-data.

get-data (and edit-data) are defined in https://datatracker.ietf.org/doc/html/rfc8526

The "get-data" command has a lot more options than other commands. Specifically:

• a datastore option must be specified (as defined in https://datatracker.ietf.org/doc/html/rfc8342).
• filtering, subtree-filtering or xpath-filtering.
• config-filter (boolean)
• origin-filters (or negated-origin-filters)
• max-depth
• with-origin