Faceting: improvement: key to filter

[etj]

Rename facet entry key to filter.

Rationale

As described in #10995, payload is

• facet is a dict:

  key	type	opt	description
  name	str	no	Name of the facet (to be used in the /facet/<facet_name> entrypoint)
  key	str	no	string to be used as KVP param in the resources query
  label	str	no	Label for the facet (could be localized)
  ...

Since key is also used elsewhere in the payload, it would be better to have different names for different purposes, so the facet info key should be renamed to filter.

The client using this info should be changed as well, so we'll probably need 2 commits:

1. add the new filter enty
2. wait a bit the client to be aligned with the new name
3. create a new commit that removes the old key entry.

[etj]

@allyoucanmap please advise when this change is reflected on the client side

[etj]

GeoNode/geonode-mapstore-client#1496