Support for Azure Entra External ID provider

[nergonvc]

Goals

1. OIDC support for microsoft's azure-b2c replacement feature called Azure Entra External ID
2. Update documentation and references for azure-b2c considered legacy and doesn't offer new integration, unless you are already a customer of the service

Non-Goals

1. This feature will not interfere or affect the azure-entra integration/provider

Background

As mentioned by Microsoft in the official documentation of Azure AD B2C and in their FAQ the Azure AD B2C goes out of sale (with an estimated support date for existing customers to atleast 2030.

Their new offering is Microsoft Entra External ID which works similarly to be honest.

Given that Next-Auth is one of the most popular authentication libraries for Javascript, it would be great to support this.

Proposal

A new provider should be added as suggested in Next-Auth Documentation, and also update the docs.

The new provider will support the new structure of claims.

Given that I've already investigated the functionality to some extend and explored also the next-auth codebase, i would be happy to contribute for this.

[benhovinga]

@nergonvc if you want a hand with implementing this, let me know.