Allow to define sources via yaml

[MRP0E]

Hello everyone,

First and foremost, thank you for your incredible work on this project! I appreciate the effort that has gone into it.

Please consider this as a preliminary draft. I encountered some challenges while attempting to deploy pgwatch without PostgreSQL as a sink. My goal was to set up pgwatch to monitor existing PostgreSQL databases while utilizing an existing Prometheus instance.

Once this PR is ready for merging, I plan to submit another PR that will introduce optional alert rules and service monitors. This will enable deployment within an existing monitoring setup that uses the Prometheus operator.

Key Changes:

• Multiple Source Definitions: Added the ability to define multiple sources through the values file.
• Prometheus-Only Deployment: Resolved the issue preventing deployment of the exporter with only Prometheus as a sink by verifying the existence of a database definition.
• Grafana Service Binding: The Grafana service is now conditionally bound to the deployment of Grafana.
• Prometheus Metrics Port: The port for Prometheus metrics is now utilized if Prometheus is created or if the Prometheus sink is enabled.

Thank you for considering this PR. I look forward to your feedback!