SCIM pagination support

Why

Pagination is described by the SCIM spec in https://tools.ietf.org/html/rfc7644#section-3.4.2.4, and is used by identity providers to check the state of GitLab's users in order to determine which user are missing or need updating.

Okta's initial test is also a request using startIndex and count, so we will be unable to support Okta until this is implemented

What

Implement SCIM compliant pagination including count/startIndex params and returning results along with itemsPerPage and totalResults

Related

Okta support: https://gitlab.com/gitlab-org/gitlab-ee/issues/11536