feat(stackable-operator): Add headless/metrics service name functions to RoleGroupRef

[Maleware]

Description

This adds standardised functions to obey https://github.com/stackabletech/decisions/issues/54 consistently across SDP

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added

[Techassi]

suggestion: Reword the doc comments.

Suggested change

	/// Set of functions to define service names on rolegroup level.
	/// Headless service for cluster internal purposes only.
	/// Returns the service name used by rolegroups for cluster internal communication only.
	///
	/// The internal use of of this service name is indicated by the `-headless` suffix.
	/// This service should not be used for communication to external services or clients
	/// and also should not be used to export metrics (like Prometheus). Metrics should be
	/// instead exposed via a dedicated service. Use [`Self::rolegroup_metrics_service_name`]
	/// instead.

[Techassi]

suggestion: Reword the doc comment.

Suggested change

	/// Headless metrics service exposes Prometheus endpoint only
	/// Returns the service name used by rolegroups to expose metrics (like Prometheus).
	///
	/// The use for metrics only is indicated by the `-metrics` suffix. This service
	/// should not be used for any internal communication or any other external
	/// communication. For internal communication, use [`Self::rolegroup_headless_service_name`]
	/// instead.

[Techassi]

note: I feel like we should rename that function to rolegroup_metrics_service_name instead.

Suggested change

	pub fn rolegroup_headless_metrics_service_name(&self) -> String {
	pub fn rolegroup_metrics_service_name(&self) -> String {

[Techassi]

Also, please add a changelog entry for these new functions.