EndpointGroupsCreateGroupOperation

Creates a user group. Its new ID and etag are returned in the result; later operations in the same batch cannot reference it. A user group name is `{"type": "PLAIN", "value": ...}` whose value is non-blank UTF-8 text of at most 256 bytes with no control characters; anything else is rejected with BAD_REQUEST. Each added member must be the ID of an existing endpoint the caller can read; a malformed ID is BAD_REQUEST, and a missing or unreadable endpoint is NOT_FOUND.