Restore Username Field template tags to Custom OAuth Config for non-nested attributes

Description:

As a RC_Admin, I would like custom oauth config to accept combination of various attributes similar to LDAP implementation so that non-nested attributes can be combined to provide username (ie #{givenName}.#{sn})

https://github.com/RocketChat/Rocket.Chat/issues/9051 removed required functionality that was added with https://github.com/RocketChat/Rocket.Chat/issues/3917.

Suggest logic be added to handle both use cases for getUserName
Here is the code that was removed for our needed use case.
https://github.com/RocketChat/Rocket.Chat/blob/c22467482123418604fe29ca9fef80e5fe0e3752/packages/rocketchat-custom-oauth/server/custom_oauth_server.js#L241

Steps to reproduce:

1. Administration->OAuth
2. Create custom oauth
3. Set Username field: #{lastname}.#{firstname}.#{uid}

Expected behavior:

getUserName able to parse identity response data with fields like { "uid": "12345", "lastname": "Jones", "firstname": "Bob" } and create Jones.Bob.12345

Actual behavior:

username is undefined with current implementation and broke deployments counting on this functionality upon upgrading to newer version.

Server Setup Information:

• Version of Rocket.Chat Server: 0.60.0 or later. Discovered on 0.69.2

Additional context

N/A

Relevant logs:

N/A

Start by reading the linked historical custom_oauth_server.js code around line 241 and trace the current Custom OAuth getUserName behavior. Reproduce the Administration→OAuth configuration with #{lastname}.#{firstname}.#{uid} and identity data containing uid, lastname, and firstname; done means it produces Jones.Bob.12345 instead of undefined.