ASP.NET Core 8 Identity Endpoints not compatible with FallbackPolicy that requires authenticated users by default

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

Currently, the recommended way to require authenticated users by default on all routes seems to be [setting the fallback policy](https://learn.microsoft.com/en-us/aspnet/core/security/authorization/secure-data?view=aspnetcore-8.0#require-authenticated-users):

```
builder.Services.AddAuthorization(options =>
{
options.FallbackPolicy = new AuthorizationPolicyBuilder()
.RequireAuthenticatedUser()
.Build();
});
```

With .NET 8's new Identity features, you can add a bunch of endpoints via `app.MapIdentityApi();`.

However, the endpoints that are supposed to be anonymous (like `/register` and `/login`) don't work correctly with the above fallback policy, because (I'm assuming) they somehow get the fallback policy applied as well, which looks like a bug to me.

Here's an example of a register call that results in an infinite redirect loop:

```
> POST /register HTTP/1.1
> Host: localhost:7100
> Content-Type: application/json
> Accept: */*
> Content-Length: 54

| {
| "email": "foo@bar.com",
| "password": "***********"
| }

< HTTP/1.1 302 Found
< Content-Length: 0
< Date: Thu, 18 Jul 2024 21:53:11 GMT
< Server: Kestrel
< Location: http://localhost:7100/Account/Login?ReturnUrl=%2Fregister

> GET /Account/Login?ReturnUrl=%2Fregister HTTP/1.1
> Host: localhost:7100
> Content-Type: application/json
> Accept: */*

< HTTP/1.1 302 Found
< Content-Length: 0
< Date: Thu, 18 Jul 2024 21:53:11 GMT
< Server: Kestrel
< Location: http://localhost:7100/Account/Login?ReturnUrl=%2FAccount%2FLogin%3FReturnUrl%3D%252Fapi%252Faccount%252Fregister

> GET /Account/Login?ReturnUrl=%2FAccount%2FLogin%3FReturnUrl%3D%252Fapi%252Faccount%252Fregister HTTP/1.1
> Host: localhost:7100
> Content-Type: application/json
> Accept: */*

< HTTP/1.1 302 Found
< Content-Length: 0
< Date: Thu, 18 Jul 2024 21:53:11 GMT
< Server: Kestrel
< Location: http://localhost:7100/Account/Login?ReturnUrl=%2FAccount%2FLogin%3FReturnUrl%3D%252FAccount%252FLogin%253FReturnUrl%253D%25252Fapi%25252Faccount%25252Fregister

> GET /Account/Login?ReturnUrl=%2FAccount%2FLogin%3FReturnUrl%3D%252FAccount%252FLogin%253FReturnUrl%253D%25252Fapi%25252Faccount%25252Fregister HTTP/1.1
> Host: localhost:7100
> Content-Type: application/json
> Accept: */*

< HTTP/1.1 302 Found
< Content-Length: 0
< Date: Thu, 18 Jul 2024 21:53:11 GMT
< Server: Kestrel
< Location: http://localhost:7100/Account/Login?ReturnUrl=%2FAccount%2FLogin%3FReturnUrl%3D%252FAccount%252FLogin%253FReturnUrl%253D%25252FAccount%25252FLogin%25253FReturnUrl%25253D%2525252Fapi%2525252Faccount%2525252Fregister

```

A workaround could be exposing a mechanism to have more control over the generated endpoints and to possibly apply `[AllowAnonymous]` to them, but I couldn't find anything to that effect, which has practically blocked me from using the above two features together.

### Expected Behavior

The generated identity endpoints that are supposed to be accessible anonymously, such as `/register`, have `[AllowAnonymous]` applied to them so they don't follow the fallback policy.

### Steps To Reproduce

_No response_

### Exceptions (if any)

_No response_

### .NET Version

8.0.202

### Anything else?

ASP.NET Core 8, following https://devblogs.microsoft.com/dotnet/whats-new-with-identity-in-dotnet-8