Add a new function to get the subject name of a given certificate

For the CreateSigningRequest method of the model PUSH (part 12 v1.05 §7.10.7), it is required to reuse tu subject name of the current certificate to create the CSR. To carry out this a new function with the declaration
SOPC_ReturnStatus SOPC_KeyManager_Certificate_GetSubjectName(SOPC_CertificateList* pCert, char** ppSubjectName, uint32_t* pSubjectNameLen)
shall be added to the KeyManager interface (src/Common/crypto/sopc_key_manager.h).

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