Support getting all user-accessible spaces by permission

[jeramysoucy]

Describe the feature: Currently there is no way to get the spaces which a user can access based on a specific capability. Contributors often want to get a list of spaces for which a user is authorized to perform some specific action, as opposed to a list of spaces that the user is authorized to see in any capacity. Solving for this means brining authorization into core, which is currently blocked on Platform Security’s re-imagining of our authorization services. The APIs we currently expose are not ideal to work with, and we don’t want to simply copy them into core as-is.

Describe a specific use case for the feature:

Example: an o11y feature that wants to enumerate the available spaces. This feature will not care, for example, about spaces where the user is only authorized for security solution.

Mock usage:

const authorizedSpaces = await spacesStart.spacesService.getAll("some capability")
for (space of authorizedSpaces) {
   const client = core.savedObjects.asScoped(request, space)
   await client.doSomething()
}

Related to #203042, where the original implementation introduced a public getSearchableSpaces function that was later abandoned because the relevant feature was only available to admins, and could rely on the global spaces list.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)