Includes:
- Extensive changes to "mapping roles" section
- New section for role mapping API
- Updates to LDAP/AD/PKI realms to refer to API based role mapping
- Updates to LDAP/AD realms: `unmapped_groups_as_roles` only looks at file-based mappings
- Updates to LDAP/AD realms: new setting for "metadata"
Original commit: elastic/x-pack-elasticsearch@6349f665f5