Extend generalized plugin resource endpoints on top of feature/issue-647 design

[Copilot]

The previous implementation started from main instead of building on top of the existing feature/issue-647_generalize-resources-plugin-endpoints branch, which already contained the proposed plugins-resources-api.yaml design spec.

Changes

• Branch alignment: Cherry-picked plugins-resources-api.yaml and its license from feature/issue-647_generalize-resources-plugin-endpoints into this branch so the implementation and design spec coexist
• API restructure (plugins-custom-resources.yaml): Replaces flat /plugins/resources?type=… with hierarchical /plugins-resources/plugins/{plugin}/types/{type}/… paths
• New discovery endpoint: GET /plugins-resources/plugins returns all plugins grouped with their available types — implemented via getPluginsWithTypes() / listPluginsWithTypes()
• Type composition: External {plugin} + {type} path segments are composed into the internal {plugin}_{type} storage key; _ is rejected in both path params to keep parsing unambiguous
• New model: PluginWithTypes (plugin name + list of types) for the discovery response
• Service layer (CompasPluginsResourceService): Added findLatestByType, findVersionsByTypeAndName, deleteByType, deleteByTypeAndName, listPluginsWithTypes
• Tests: Unit tests for all new and modified resource/service methods

Note

PR base should be changed to feature/issue-647_generalize-resources-plugin-endpoints to correctly scope the diff to implementation-only changes on top of the existing design.