Add terminals to the Kubernetes deployment service

app/controllers/projects/environments_controller.rb

app/models/concerns/reactive_caching.rb

app/models/environment.rb

app/models/project_services/deployment_service.rb

app/models/project_services/kubernetes_service.rb

app/serializers/environment_entity.rb

lib/gitlab/kubernetes.rb

lib/gitlab/workhorse.rb

spec/controllers/projects/environments_controller_spec.rb

spec/factories/projects.rb

spec/lib/gitlab/kubernetes_spec.rb

spec/lib/gitlab/workhorse_spec.rb

spec/models/environment_spec.rb

spec/models/project_services/kubernetes_service_spec.rb

spec/support/kubernetes_helpers.rb