Allow simple client side update of user profile / attributes

## Problem

Currently, the only way to update user profiles (standard/custom attributes is via Admin API, which is server side)

In many cases it would be helpful for Client to be able to directly update from Client device.

## Appetite

6 weeks.

## Solution

The solution need to be designed technically, however, it should:
1. Have a client side RESTful API, based on access token or cookies.
2. Have SDK supports
3. Integrate with access control of attributes.

## Success definition

> Based on the “Problem” statement, define how we know if the feature is successful using event tracking.
>
> Event definition: Event name, Action that triggered the event, Which page the user will perform this action, Event properties

## Rabbit holes

> Details about the solution worth calling out to avoid problems

## No-goes

> Functionality or use cases we intentionally aren’t covering to fit the appetite

Review the existing Admin API, client SDK support, and attribute access-control design before defining the client-side RESTful flow. Done means a technical design covering access tokens or cookies, SDK support, attribute permissions, and event-tracking success criteria; the issue names no files or tests to run.

Written by the indexing model from the issue text.