Add basic components for Tracing UI

app/assets/javascripts/observability/client.js

...ipts/observability/components/observability_container.vue

...s/javascripts/observability/components/skeleton/index.vue

app/assets/javascripts/observability/mock_traces.json

app/assets/javascripts/pages/projects/tracing/index/index.js

app/assets/javascripts/tracing/components/tracing_list.vue

app/assets/javascripts/tracing/list_index.vue

app/controllers/projects/tracing_controller.rb

app/helpers/projects/observability_helper.rb

app/views/projects/tracing/index.html.haml

lib/gitlab/observability.rb

spec/frontend/observability/observability_container_spec.js

spec/frontend/observability/skeleton_spec.js

spec/frontend/tracing/list_index_spec.js

spec/helpers/projects/observability_helper_spec.rb

spec/lib/gitlab/observability_spec.rb

spec/requests/projects/tracing_controller_spec.rb