[jampukka]

Closes #203. A fourth configurable link source for the top-level links of /collections (JSON and HTML), alongside the three from #177.

• api.collections.links.*
  • Same shape as api.links.*: api.collections.links=a,b plus <name>.href/rel/type/title (hreflang optional)
  • Deliberately not the issue's collections.links=a,b,c: that would collide with a collection named links
• HakunaConfigParser.readCollectionsAdditionalLinks() / FeatureServiceConfig.collectionsAdditionalLinks
  • Reuses readLinksWithPrefix; the Localized* wrapper delegates to the wrapped service (links are not translated)
• collections.ftl
  • New "Additional Resources" section listing the response's links except self and alternate, which the page already renders itself

To note:

• Fully opt-in
• A configured link missing any of href/rel/type/title fails startup, as with the other three sources
• A configured link with rel=self or rel=alternate appears in the JSON but not in the HTML section; fixing that needs a change to CollectionsContent, left out on purpose
• docker/cfg/addresses.properties now shows three such links: the INSPIRE-required describedby + application/xml metadata record, the recommended text/html variant of the same record, and a license link.