Certificate Forwarding Middleware Does Not Support URL-Encoded PEM Certificates

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

The certificate forwarding middleware only parses base64-encoded PEM certificates. When the certificate is URL-encoded (as is standard with some reverse proxies), it fails to parse the certificate and populate HttpContext.Connection.ClientCertificate. This limits compatibility with common reverse proxies that perform mutual TLS and URL-encode the client certificate in headers.

I think it is this function that would need to be updated to return the certificate from both a url encoded or base64 encoded PEM certificate in the specified header. https://github.com/dotnet/aspnetcore/blob/d382ae48752a70264b32d9d8610f6ad5e71e1bcf/src/Middleware/HttpOverrides/src/CertificateForwardingOptions.cs#L27

### Expected Behavior

The middleware should correctly parse and return the client certificate from the header for both base64-encoded and URL-encoded PEM certificates in the specified header.

### Steps To Reproduce

- Configure a reverse proxy (e.g., Azure Application Gateway or AWS ALB) to perform mutual TLS and forward the client certificate in a header.
- Deploy an ASP.NET Core application using the Certificate Forwarding Middleware.
- Observe the behavior when the forwarded certificate is URL-encoded.

### Exceptions (if any)

_No response_

### .NET Version

8.0.410

### Anything else?

_No response_