[4.1][webservices] post user fix editable on frontend only

[alikon]

Pull Request for Issue #36936 .

Summary of Changes

if editable only on frontend allow the post from the webservices like from admin

Testing Instructions

Set a User->Field (Phone as an example) to 'Required'="yes", AND 'Editable In'="Site"

Create a user with REST API:
POST: {{base_url}}/{{base_path}}/users

BODY:
{"block":"0","email":"bob.smith@test.com","groups":["2"],"lastResetTime":"","lastvisitDate":"","name":"Bob Smith","params":{"admin_language":"","admin_style":"","editor":"","helpsite":"","language":"","timezone":""},"password":"qwerty1235%%##","password2":"qwerty1235%%##","registerDate":"","requireReset":"0","resetCount":"0","sendEmail":"0","username":"bobsmith"}

Actual result BEFORE applying this Pull Request

error
"title": "Invalid field: phone"

Expected result AFTER applying this Pull Request

able to create user

[nbradshaw45]

Test passed