targetRef matching API improvements

[slonka]

Description

It's not always clear how to call functions like MatchedPolicies and what to do with the results without digging into implementation. For single item rules sometimes we call .Compute on it like here

kuma/pkg/plugins/policies/meshproxypatch/plugin/v1alpha1/plugin.go

Line 41 in 4b04a60

rule := policies.SingleItemRules.Rules.Compute(core_xds.MeshSubset())

(and why is MeshSubset hardcoded not taken from targetRef?) and sometimes we just get the Rules[0]

kuma/pkg/plugins/policies/meshtrace/plugin/v1alpha1/plugin.go

Lines 134 to 135 in 60024ab

	rawConf := rules.Rules[0].Conf
	conf := rawConf.(api.Conf)

- which one is correct? what about From/To rules?

The algorithm is nicely documented on the website https://kuma.io/docs/2.5.x/policies/targetref/#understanding-targetref-policies but I think we could do a better job explaining in code what's what and how to use it.

Tagging @lobkovilya because he probably knows best.

[michaelbeaumont]

Maybe we can structure the types such that you can't use both

[jakubdyszkiewicz]

xref #8155

Triage: both ways are correct. We could unify those occurrences of the code.
Also consider documenting .Compute function.

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If you think this issue is still relevant, please comment on it or attend the next triage meeting.

[github-actions]

This issue was inactive for 90 days. It will be reviewed in the next triage meeting and might be closed.
If you think this issue is still relevant, please comment on it or attend the next triage meeting.

[github-actions]

This issue was inactive for 90 days. It will be reviewed in the next triage meeting and might be closed.
If you think this issue is still relevant, please comment on it or attend the next triage meeting.