wip: graceful shutdown

[jonaro00]

Description of change

• cargo-shuttle
  • Sends a SIGINT to runtime child process if a SIGINT was received. Allows local run to use graceful shutdown like when deployed.
  • TODO: Windows
• runtime
  • Call service's shutdown when process was interrupted (SIGINT)
• service
  • Adds Clone trait bound on Service
  • Adds shutdown() trait method to optionally implement for a Service

Partially implements #2004

How has this been tested? (if applicable)