ClientCertificate property no longer triggers renegotiation for HttpSys

## ClientCertificate property no longer triggers renegotiation for HttpSys

The `HttpContext.Connection.ClientCertificate` property will no longer trigger TLS renegotiations for HttpSys. See https://github.com/dotnet/aspnetcore/issues/34124 for discussion.

### Version introduced

6.0

### Old behavior

Setting `HttpSysOptions.ClientCertificateMethod = ClientCertificateMethod.AllowRenegotation` allowed renegotiation to be triggered by both `HttpContext.Connection.ClientCertificate` and `HttpContext.Connection.GetClientCertifiateAsync`.

See https://github.com/aspnet/Announcements/issues/422 for related changes in 5.0.

### New behavior

Setting `HttpSysOptions.ClientCertificateMethod = ClientCertificateMethod.AllowRenegotation` will allow renegotiation to be triggered only by `HttpContext.Connection.GetClientCertifiateAsync`. `HttpContext.Connection.ClientCertificate` will return the current certificate if available, but will not renegotiate with the client to request one.

### Reason for change

When implementing the same features for Kestrel it became clear that applications needed to be able to check the state of the client certificate before triggering a renegotiation. This enables the following usage pattern to deal with issues like the request body conflicting with the renegotiation:

```
if (connection.ClientCertificate == null)
{
  await BufferRequestBodyAsync();
  await connection.GetClientCertificateAsync();
}
```

### Recommended action

Applications that use delayed client certificate negotiation need to call GetClientCertificateAsync() to trigger that.

### Category

ASP.NET

### Affected APIs

 [HttpSysOptions.ClientCertificateMethod](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.server.httpsys.httpsysoptions.clientcertificatemethod)
  [HttpContext.Connection.ClientCertificate](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.http.connectioninfo.clientcertificate)
  [HttpContext.Connection.GetClientCertifiateAsync](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.http.connectioninfo.getclientcertificateasync)



---
#### Issue metadata

* Issue type: breaking-change

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ClientCertificate property no longer triggers renegotiation for HttpSys #466