feat(posthog-server): Automatic configuration of PostHog client

### Description

Allow the PostHog client to automatically be configured via a properties file _or_ environment variables, e.g.,

_posthog.properties_
```
posthog.api-key=phc_abc123
posthog.host=https://ph.example.com
posthog.local-evaluation=true
posthog.poll-interval-seconds=15
```

