Users service

Bitrix method	Enum method	API
`user.fields`	`Method.USER_FIELDS`
`user.current`	`Method.USER_CURRENT`
`user.add`	`Method.USER_ADD`
`user.update`	`Method.USER_UPDATE`
`user.get`	`Method.USER_GET`
`user.search`	`Method.USER_SEARCH`