Add ApiExtension ServiceLoader extension point

[jampukka]

Nothing in the repository implements it yet beyond a test fixture. OGC API - Tiles (later PR) is the first planned consumer. The Features resources are not moved behind it.

• Extension discovery

  • Found with java.util.ServiceLoader, as OutputFormatFactorySpi
  • A module is enabled by putting its jar on the classpath and configuring it
  • No order is imposed on extensions, since links, conformsTo and collections are unordered in OGC API Features

• ApiExtensionFactory

  • create(ApiExtensionContext) runs once at startup. Returning null means nothing is configured for it and it is skipped with a log line. Throwing fails startup, so a broken configuration cannot serve a partial API

• ApiExtensionContext

  • Record of the config parser, config path and feature collections. The first record in hakunapi-core
  • One argument rather than three, so adding something later is not a breaking change for factories (same reason as OutputFormatFactorySpi.create(Map))

• ApiExtension

  • One instance describes everything the servlet layer publishes for its module: conformance classes, operations, landing page and collection links, non-feature collections, f values and injectables
  • getInjectables() returns the module's own config objects for DI, declared as plain types so core has no DI framework
  • getCollectionIds is separate from getOperations because the OpenAPI generator asks a freshly constructed operation that has no injected state
  • getFormatMediaTypes makes an extension's f values work like f=json. Core's own ids are resolved first and win
  • Shared across requests, so thread-safe. Closeable, closed by the listener with the context

• CollectionInfo

  • setItemType (new) lets an extension publish a collection that is not a feature collection. The extension builds the whole CollectionInfo, since only it knows the links, extent and item type

• OpenAPISchemaContributor / OpenAPISchemaContributors

  • Second ServiceLoader SPI, letting an extension describe its own response models without the generator naming them. First non-null answer wins

• The 400 for an invalid f= now lists every accepted value: metadata formats, configured output formats and extension formats. It previously listed only the metadata formats, so clients matching on the old message text will see a different list.

• With no extension on the classpath nothing else changes.