Add additional links to /collections response

[jampukka]

Support for configuring additional links was added as part of #177.

FeatureServiceConfig.additionalLinks (config key api.links.*) feeds the landing page. Per-collection links (default.collections.links.* + <collection>.links.*) feed each entry in /collections' collections[].links. But the top-level links array of the /collections response itself has no configurable additions - CollectionsMetadataImpl never reads any "additional links" source.

OGC API Features Core (Part 1) recommends links to additional resources from /collections. INSPIRE Best Practices turns that recommendation into a requirement for metadata record.

Proposed change:

• New link source for /collections top-level links. Suggested key: collections.links.* (parallel to api.links.* and default.collections.links.*)
• Extend HakunaConfigParser with reader, field + getter/setter on FeatureServiceConfig, write in CollectionsMetadataImpl alongside existing self/alternate links