The native roles store previously would issue a search if attempting to
retrieve more than a single role. This is fine when we are attempting
to retrieve all of the roles to list them in the api, but could cause
issues when attempting to find roles for a user. The search is not
prioritized over other search requests, so heavy aggregations/searches
or overloaded nodes could cause roles to be cached as missing if the
search is rejected.

When attempting to load specific roles, we know the document id for the
role that we are trying to load, which allows us to use the multi-get
api for loading these roles. This change makes use of the multi-get api
when attempting to load more than one role by name. This api is also
constrained by a threadpool but the tasks in the GET threadpool should
be quicker than searches.

See elastic#33205