Summon can notify child process of secrets changes

[sgnn7]

Currently:

• We don't have a way to notify the child process of changes to secrets
• Summon has no way to poll secrets for those changes either

Since not all use cases can use Secretless Broker, this could be valuable to add to the code even if it has to do it in some sub-optimal ways (polling, SIGHUP).

Describe the solution you would like

• Summon can determine when a secret configured has changed (polling is fine here)
• Summon can notify parent process to refresh its secrets
  • For @SUMMONENVFILE, some signal would be fine (e.g. SIGHUP)
  • For env var changes, child process needs to restart (SIGKILL) so we may need a configurable signal to use.
  • (optional) Investigate if symlinks would work on Windows which would make this easier (my best guess is that they don't work OOTB).
• (optional) Static or passthrough "live" healthcheck endpoint in summon
• See Is secret Rotation/Update managed? #132 for more details

Additional context

Further conversation on this can be found in #132.