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/src/cdm_session.cpp

@@ -1302,6 +1302,18 @@ bool CdmSession::HasRootOfTrustBeenRenewed() {
   return true;
 }
 
+CdmResponseType CdmSession::LoadCastPrivateKey(
+    const CryptoWrappedKey& private_key) {
+  return crypto_session_->LoadCertificatePrivateKey(private_key);
+}
+
+CdmResponseType CdmSession::GenerateRSASignature(const std::string& message,
+                                                 std::string* signature,
+                                                 RSA_Padding_Scheme scheme) {
+  return crypto_session_->GenerateRsaSignature(message, signature,
+                                               scheme);
+}
+
 // For testing only - takes ownership of pointers
 
 void CdmSession::set_license_parser(CdmLicense* license_parser) {