Add New Relic Deployment Marker Support to action-deploy-to-pantheon

## Background

Teams are currently missing valuable deployment visibility in New Relic monitoring unless they instrument this individually. New Relic offers a deployment marker GitHub Action that can track changes, but integrating this functionality directly into our deployment action would provide a more seamless workflow for teams deploying to Pantheon environments.

### Use Case

As a developer deploying applications to Pantheon environments, I want our deployment action to automatically notify New Relic about successful deployments so that I can correlate performance changes with specific code deployments without requiring additional manual steps or separate GitHub Actions in my workflow.

## Acceptance Criteria

• The action accepts optional New Relic configuration parameters (API key, application ID, opt. deployment description)
• Deployment markers are sent to New Relic API only when New Relic parameters are provided
• The action continues to function normally when New Relic parameters are omitted (backward compatibility)
• Successful deployment marker creation is logged for visibility
• Failed deployment marker requests do not cause the overall deployment action to fail
• Documentation includes examples of how to configure New Relic deployment tracking
• The implementation follows New Relic's recommended deployment marker API patterns