Allow changing the header used for certificate auth

[TimHess]

Steeltoe v4 defaults to using the header X-Client-Cert for sending and receiving certificates used for certificate authorization (thus effectively locking it in for authentication as well) without any ability to change the header name.

I think it is possible this could be changed in an application by configuring CertificateForwardingOptions and an HttpClient separately, but it would also be trivial to expose parameters to allow callers to set the value via the Steeltoe extensions linked above.