Add securitySchemes and security requirements for endpoints created by MapIdentityApis

### 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.

An application that uses the new `MapIdentityApis` extension method in .NET 8 is adding ASP.NET Identity as an AuthN/AuthZ scheme in their application. But this auth scheme is not reflected in OpenAPI documented generated by the (new) built-in ASP.NET OpenAPI document generation feature. Some users will want to see this information reflected in the OpenAPI doc for their application.

### Describe the solution you'd like

The `MapIdentityApis` extension method should either automatically add the appropriate security schemes and security requirements metadata or provide a simple means to add this optionally.

### Additional context

_No response_