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_engine.h

@@ -388,6 +388,13 @@ class CdmEngine {
   virtual void SetDefaultOtaKeyboxFallbackDurationRules();
   virtual void SetFastOtaKeyboxFallbackDurationRules();
 
+  // A signing method specifically used by Cast.
+  // This method should not be used otherwise.
+  virtual CdmResponseType SignRSA(const std::string& wrapped_key,
+                                  const std::string& message,
+                                  std::string* signature,
+                                  RSA_Padding_Scheme padding_scheme);
+
  protected:
   friend class CdmEngineFactory;