Liveness / Readiness probes for `PythonWatchManager`

[gabe-l-hart]

Is your feature request related to a problem? Please describe.

As an operator maintainer in an application, I need to be able to probe the liveness/readiness of the operator. With the AnsibleWatchManager, the native /healthz and /readyz HTTP endpoints were accessible, but there's no equivalent with PWM.

Describe the solution you'd like

Add a heartbeat_thread to the PWM's group of managed threads. This thread will periodically dump the current time to a file if enabled.

Describe alternatives you've considered

• Use a lightweight HTTP server to expose GET endpoint(s). The downside to this is that adding an HTTP server is non-trivial to code and adds non-trivial dependencies to do in a robust way.
• Require users to install procps in their docker builds and run a ps aux | grep ... style check to ensure the operator is running. This has the downside of requiring a potentially dangerous tool (ps can be used to probe running processes), and it is also a bad approximation for the state of the running operator code.