Add subscription mechanism for install phases covered by verify

[vaibhavdaren]

Background

The prepare/artifact plugin's verify phase injection currently uses a static set of phase names to decide which tmt-managed install phases should be covered by verification (source):

_tmt_install_phase_names = {
    'essential-requires',
    'requires',
    'recommends',
    'requires (dist-git)',
    'recommends (dist-git)',
}

This list is hard to maintain: every time a new source of tmt-injected install phases is added (e.g. a new dist-git variant, a new built-in prepare plugin), the artifact plugin needs a matching update.

Proposed improvement

Add a lightweight subscription/registration mechanism so that plugins which inject install phases on behalf of tmt can declare themselves as candidates for artifact verification, instead of the artifact plugin hard-coding their names.

A simple approach could be a class-level marker or a registry call on PrepareInstall subclasses/instances — similar to how @provides_method already works for plugin discovery.

The mechanism should remain simple. The goal is not to build a general-purpose event system, but to remove the need for the static name list while keeping the intent clear.

Notes

• Not a blocker — the current static list works for all known cases today.
• Ideally the solution would also cover any future case where all tmt-injected install phases end up being verified, making the opt-in the default rather than the exception.

[AthreyVinay]

From triage - another approach would be to set a flag rather than subscription. Maybe give it a try?