Provide option for graceful shutdown

[lgadban]

Is your feature request related to a problem? Please describe.
Currently there is no option for a graceful shutdown.
When using "external" mechanisms for health checking (i.e. not relying on Readiness or Endpoints from k8s) there is no way to start failing health checks, gracefully terminate existing connections, prevent new incoming connections, etc.

Describe the solution you'd like
Some combination of scripted health check failures, listener draining, configurable terminationGracePeriodSeconds on the proxy deployment. Ideally exposed by a single helm value (?)

Describe alternatives you've considered
Some of the above mentioned solutions could be implemented after the Gloo installation

Additional context
Similar discussion in #1084
Ideally, with a graceful shutdown, ALBs relying on solely health checking envoy instances can perform e.g. an in-place upgrade with minimal-to-none downtime