GPDS backend migration

(Architecture inspired from data-discovery, Code imported and adapted from legacy unified-interface)

• Add spring boot backend (to the latest version)
• Add gradle build script
• Add GitLab CI backend test job
• Add spring cloud configuration
• Add spring actuator & security
• Add Elasticsearch REST client
• Import code from unified-interface (adapt it to latest spring, junit and ES client)
  • BrAPI endpoint API
  • GnpIS GPDS data lookup API (result page with facets)
  • GnpIS XRef API
  • Handling private context when receiving HTTP Authorization by fetching the user group and requesting the corresponding private ES aliases
• Configure a single JAR to run frontend and backend (with all REST APIs)
• Update to BrAPI v1.2 specification (almost completely)
• Add BrAPI documentationURL and schema:includedInDataCatalog fields (Elixir use case)