Add a Serializable Credential Class

[mcstoer]

The credential contains the

• Version: Class version
• Flags: Indicates if there is a label attached
• Credential Key: Indicates the type of credential by vdxfkey, including username or password
• Credential: The credential itself
• Scopes: Indicate the ID or URL that is supposed to receive the credential
• Label (optional): Any additional information related to the credential