feat(v2/app): Add application lifecycle coordinator with component registry

Context

Part of the LabKit v2 package decomposition from the UAM PoC (!314).

Tracking issue: gitlab-org/quality/quality-engineering/team-tasks#4286 (closed) Parent epic: gitlab-org/quality#360

What's in this MR?

• v2/app package: central application lifecycle coordinator
  • Pluggable Component interface (Start(ctx) error + Shutdown(ctx) error)
  • Ordered startup, reverse-ordered graceful shutdown
  • Threads shared logger and tracer to all registered components
  • NewForTesting constructor for in-process test doubles
• v2/testing/apptest package: test helper that creates a pre-configured App with in-memory logger and trace recorder

Dependencies

Depends on already-merged v2/log and v2/trace packages.

Related

• gitlab-org/quality/quality-engineering/team-tasks#4293 (follow-up: graceful degradation on startup for non-critical components)