Improve Admin API for User Profiles update (standard/custom attributes)

## Problem

Currently, the standard/custom attributes related GraphQL Admin API, could only be updated as a whole, as the backend implementation is a JSONB.

It created the following problems:
1. Client requires at least 2 requests to update an attribute (one for query, one for storing the updated attribute)
2. It has a high chance for conflict as updating one attribute will update all attributes no matter what. There is no way to guarantee consistency.

## Appetite

2 weeks

## Solution

A new Admin API should be available for updating attributes; it should:

1. Allow updating one attribute without updating all.
2. Provide ways to conduct C*UD, aka create a new attribute or delete an attribute, besides updating one.
3. Provide an "update only if" option for simple consistency (optional, welcome for a better idea.)

Start by tracing the existing GraphQL Admin API for standard and custom user-profile attributes and the backend JSONB storage path. Define the API behavior for individual create, update, and delete operations, including the optional update-only-if condition. Done means clients can change one attribute without reading and rewriting the full attribute set, with consistency behavior covered.

Written by the indexing model from the issue text.