Exposing the Cast Signing Algorithm

1. Exposing the Cast Signing Algorithm in cdm core.
2. Update core Cast tests to use new core CDM Cast signing API.

Bug: 279671867
Bug: 279672538
Change-Id: Ia73c4b5e6dd61edf790bca97a321881d310e7a99

libwvdrmengine/cdm/core/include/cdm_session.h

@@ -219,6 +219,13 @@ class CdmSession {
 
   virtual metrics::SessionMetrics* GetMetrics() { return metrics_.get(); }
 
+  virtual CdmResponseType LoadCastPrivateKey(
+      const CryptoWrappedKey& private_key);
+
+  virtual CdmResponseType GenerateRSASignature(const std::string& message,
+                                               std::string* signature,
+                                               RSA_Padding_Scheme scheme);
+
  private:
   friend class CdmSessionTest;