feat: Optional support for updating configuration at runtime

[akanieski]

It would be great to be able to adjust the MINIMUM_AGENT_COUNT at runtime without have to restart the agent orchestrator service. It may also be useful to change other configurables at runtime.

Proposal
Replace get properties in KuberenetesAgentHostService and ACIAgentHostService for configurables like MINIMUM_AGENT_COUNT so that they first attempt to read from a path on the filesystem. This would give people the opportunity to use Kubernetes ConfigMap objects to adjust values at runtime. You could for example, scale your minimum agent count from a high number during busy periods, and scale down to less during slow periods.