FizzClientContext::setClientCertificate() signature change

Summary:
- we should be able to do something like:

```
FizzClientContext::setClientCertificate(other.getClientCertificate());
```

Reviewed By: knekritz, sharmafb

Differential Revision: D54093188

fbshipit-source-id: d103554434130020777eb8d6a8c2463e86e1d5f2

fizz/client/FizzClientContext.h

@@ -112,7 +112,7 @@ class FizzClientContext {
   /**
    * Sets the certificate to use if the server requests client authentication
    */
-  void setClientCertificate(std::shared_ptr<SelfCert> cert) {
+  void setClientCertificate(std::shared_ptr<const SelfCert> cert) {
     clientCert_ = std::move(cert);
   }