Default theme / branding for Freemarker templates (respecting CSP header restrictions)

[aaime]

OGCAPI makes use of free marker templates that are embedded into jars. We also have the generic GeoServer theme extension point (that adds header contributions which can be used to override CSS - and thus override anything).

While the user can configure free marker templates for OGCAPI (and GetFeatureInfo)...

We do not have a programatic way to manage this:

• What is emended in the original jars is defaults used (yes user can configure in data directory, but not developer)
• To respect CSP restrictions it would be nice to have a similar ability to add to inject in the header as is available in wicket

I would like to combine these two points so that I can make a jar to supply overrides for OGCAPI content while respecting CSP header policy.