Exposing the Cast Signing Algorithm

[ Merge of http://go/wvgerrit/178076 ]

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
Test: com.google.android.wvts
Change-Id: Ia73c4b5e6dd61edf790bca97a321881d310e7a99
(cherry picked from commit 8996b624d73adf14db45e54653e7ddc513f3895c)

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -394,6 +394,13 @@ 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;