[FEAT]: API - Ease "username" for user management via API

[brainboutique]

What would you like to see?

Background: Using Simple SSO - for example with external authentication service like Azure AAD - requires to link a anythingllm user with an user "owned" by the authentication service.
In most cases it is not possible to store the anythingllm's user id in that directory.

Proposed change:

• Add "GET /v1/admin/usersByName/{username}" to Create or Update the user identified by that username.
• Add "PATCH /v1/admin/usersByName/{username}" to Create or Update the user identified by that username.

PS: Alternatively, as the DB apparently is already ensuring uniqueness of username (as it should), refactor existing API to use the user-defined username for all API flavours.