Extensibility for Identity Endpoints (MapIdentity<TUser>)

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

_No response_

### Describe the solution you'd like

After reading about the [Identity API endpoints](https://devblogs.microsoft.com/dotnet/asp-net-core-updates-in-dotnet-8-preview-4/#identity-api-endpoints), I immediately got excited, tried it and was able to make it work after 10 minutes! (Yes! All identity features almost instantly created and working).

One thing came to mind is on up to what extent these endpoints are extensible (e.g. Decorator pattern) to support custom workflow mostly for registration or login?

I [explored the code](https://source.dot.net/#Microsoft.AspNetCore.Identity/IdentityApiEndpointRouteBuilderExtensions.cs,32) that creates the routes and it seems the behavior are directly added to the routes pipeline. For instance, the RegisterRequest DTO contains only Email and Password. Normally, you would like to throw in a few more fields captured in your registration process.

If I missed anything that relates to this feature, I would appreciate if somebody can point them out to me.

### Additional context

_No response_