VII: Add external login support

[shadowfoxish]

The demo in this sample is fantastic, and was 90% of the way there for what I needed. Not using ASPNET Identity was a requirement for me, since I have an existing database of users I need to authenticate locally with, but also support external federated authentication.

However, I could not find a complete sample that demonstrates this particular workflow anywhere (and didn't use ASPNET Identity). This took me the better part of the week to learn OpenID Connect, refresh on MVC, and to figure out and wrap my head around OpenIddict. So, hopefully to save the next dev some time, I tender for your consideration, part VII; Adding an external login provider.

In this PR, I updated the OpenIddict packages to .NET 8 which is the current LTS .NET release from Microsoft, and updated the OpenIddict packages to latest and greatest.

The key changes are:

• Adding an OpenIddict Client in Startup.cs, which includes the Microsoft Entra provider (or which ever one you want)
• Adding a new route in the AccountController for LoginEntra that returns Challenge, which is the magic sauce for having OpenIddict automagically authenticate the request with the external provider.
• Adding a login callback to AuthorizationController, which handles the post-back from Entra after the user successfully authenticates. This is taken directly from the official OpenIddict documentation
• Add a button on the home page and login page to ask a user to sign in using Entra.

[robinvanderknaap]

Thank you for your pull request. I think it's a great idea to expand the article with external login providers. I'm currently on holiday in the US, in about three weeks I will be back home and will be able to take a closer look. Thanks again!