Fix ReceiverProtocols confusion

[PietroPasotti]

          in that case we have some confusion in the types and enums around the code that we NEED to fix. See the ReceiverProtocol enum

Originally posted by @PietroPasotti in #7 (comment)

[PietroPasotti]

task breakdown:

• verify what the actual possible configuration options for the receivers block can be.
  Can we put in there any one of the contrib receivers as listed in https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main ?
• Decide on a list of protocols we want to enable
• adjust force-enable options accordingly
• adjust tracing charm lib (the ReceiverProtocols enum) and document clearly our choice next to it
  • no changes should be necessary for tempo-cluster
• adjust receiver ports in https://github.com/canonical/tempo-coordinator-k8s-operator/blob/main/src/tempo.py#L49
• propagate this change down to grafana-agent

[michaeldmitry]

ADR:
canonical/observability#196

Fixed by:
canonical/tempo-worker-k8s-operator#11
#13
canonical/tempo-k8s-operator#149