V14 External login linking + Proposed error handling

[Migaroez]

Prerequisites

• I have added steps to test this contribution in the description below

Description

This PR introduces some proposed helper methods to uniformly communicate external login errors that come into the backend trough the configured callback methods to a specific frontend path.

Example code

ExampleGithubCode.zip (requires installation of AspNet.Security.OAuth.GitHub)
This adds all the necessary bits to register github as an external login provider and some bad frontend code to link/unlink.

When starting the linking process from the backend, you will need to open your devtools and click on the CORS blocked link manually to continue, the Frontend team has a task to improve this behaviour

If you use the endpoints, make sure to set the Umbraco authentication cookie as it is needed to figure out what user the linking process should complete on when returning trough the callback.

Updated example registration code

backOfficeAuthenticationBuilder.AddGitHub(
                        backOfficeAuthenticationBuilder.SchemeForBackOffice(GitHubBackOfficeExternalLoginProviderOptions
                            .SchemeName)!,
                        options =>
                        {
                            options.SetUmbracoBasedCallbackPath("signin-github");
                            options.ClientId = "d261ea94c7bf0255801d";
                            options.ClientSecret = "88e795ccb1050c2806dd9d87ec0c6e2048f1a6a4";
                            options.Scope.Add("user:email");
                            options.SetDefaultErrorEventHandling(GitHubBackOfficeExternalLoginProviderOptions
                                .SchemeName);
                        });