[ogcapi-records] keywords facet should split keywords to list

[pvgenuchten]

Description

at the moment the keywords facet does not manage the case if the column contains multiple keywords
the combination of keywords is returned with a count, expected is that the list is split to individual keywords and keywords are counted again on occurence.

now:

tree: 5
leaf: 6
tree,leaf: 2

expected

tree: 7
leaf: 8

Screenshot: