why the cert must be provided as well if private key is already present in options

[MingZhang-PS]

Description

In my understanding, the "key" in options is used as private key to sign the assertion, but what's the purpose of "cert"? For me, it doesn't make sense to have "cert" as a mandatory option https://github.com/auth0/node-saml/blob/master/lib/saml20.js#L62

Environment

Please provide the following:

• **Version of this library used: saml2.0
• **Version of the platform or framework used, if applicable: N/A
• **Other relevant versions (language, server software, OS, browser): nodejs
• **Other modules/plugins/libraries that might be involved:N/A