Feature: Warmup for controllers

[sbueringer]

Tasks

• [v0.21] Design: 📖 Add a design for supporting warm replicas. #3121
• [v0.21] Initial implementation: ✨ Implement warm replica support for controllers #3192
• Further improvements:
  • Metrics: 📖 Add a design for supporting warm replicas. #3121 (comment)
  • [TBD] util func for health/readiness checks: 📖 Add a design for supporting warm replicas. #3121 (comment)
  • (Check proposal if there is more)

[sbueringer]

@godwinpang @alvaroaleman

Let me know if I should add additional tasks to this umbrella issue