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 https://github.com/canonical/tempo-coordinator-k8s-operator/pull/7#discussion_r1660908400_

[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