Create a new user.

Password confirmation fields (passwordConfirmation, linuxPasswordConfirmation,
windowsPasswordConfirmation) are optional for API callers. If a password is supplied
and the matching confirmation key is omitted entirely, the API auto-confirms it.
An explicitly provided confirmation value (including empty string) is validated as-is.

Tenant may be selected with query accountId or tenantId (master-tenant users with
admin-accounts only). Nested route POST /api/accounts/{accountId}/users is equivalent.