Account for inherited runners in RunnerPolicy

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR extends the :read_runner runner policy rule so that it is enabled also for runners that are inherited by a user (i.e. runners that the user should be able to view when in the group runners admin view). An example:

The user is the owner of group, which is set up to use shared runners. Without this MR, querying for user.can?(:read_runner, instance_runner) would return false. With this change, as long as user is an owner of group, user.can?(:read_runner, instance_runner) will return true.

This logic is required for !96770 (merged) (see !96770 (comment 1088883167)).

Part of Group runners should display all runners availa... (#337838 - closed)

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This MR fixes the scenario in the bottom-right of the picture (instance runners should be listed there in addition to the runners from descendant scopes):

	DESCENDANTS	ALL
root user	✅	✅
group owner	✅	❌

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