add possibility to set a bearer token secret in ServiceMonitor

[andibraeu]

This PR adds support for bearer token authentication in the ServiceMonitor configuration. This enhancement allows users to secure their Prometheus metrics endpoint with token-based authentication.

Changes made:

• Added bearerToken configuration with secretName and secretKey properties to the serviceMonitor section in values.yaml
• Updated the ServiceMonitor template to include bearer token authentication when configured
• Added documentation in the README explaining how to use the bearer token feature
• Created a test configuration to validate the implementation

Example usage:

serviceMonitor:
  enabled: true
  scrapeInterval: 30s
  labels:
    release: prometheus
  bearerToken:
    secretName: "prometheus-token"
    secretKey: "token"

This enhancement improves security for Home Assistant metrics collection while maintaining backward compatibility with existing configurations.

[andibraeu]

Hi @pajikos
can you review this PR, please?

[pajikos]

Hi @andibraeu, thank you for both of your PRs. Today I returned from vacation.