Add ClientIdentifier in RequestCommon

Summary:
What?
- ClientIdentifier = Hash string of primary (non-host) tls client identities
- **This diff** adds ClientIdentifier field in RequestCommon which will be extracted in codegen (next diffs) to pass the ClientIdentifier in header to memcache server
- This change is ONLY for proxies and will be executed during shadow testing ONLY

WHY?
- SAP and uCache are working on securing look-aside cache called "Key Client Binding" feature where we bind keys in uCache with ClientIdentities. This ensures that cached data is restricted to the specific client who has access to the backed service.

- Design doc: https://fburl.com/gdoc/psjwp58j

Reviewed By: lenar-f

Differential Revision: D62452894

fbshipit-source-id: 4dfb17a4b3d3389b610879304ce1c304838aa034

third-party/mcrouter/src/mcrouter/lib/carbon/RequestCommon.h

@@ -83,6 +83,14 @@ class RequestCommon : public MessageCommon {
     return cryptoAuthToken_;
   }
 
+  const std::string& getClientIdentifier() const noexcept {
+    return clientIdentifier_;
+  }
+
+  void setClientIdentifier(folly::StringPiece clientIdentifier) noexcept {
+    clientIdentifier_ = clientIdentifier.str();
+  }
+
  protected:
   void markBufferAsDirty() {
     serializedBuffer_ = nullptr;
@@ -92,6 +100,8 @@ class RequestCommon : public MessageCommon {
   const folly::IOBuf* serializedBuffer_{nullptr};
   // cat token(s) in string serialzed format
   std::optional<std::string> cryptoAuthToken_;
+  // Hash string of primary (non-host) tls client identities
+  std::string clientIdentifier_;
 };
 
 } // namespace carbon