cert is reworked to be optional, since it is not mandatory for xml signature library

[MingZhang-PS]

Description

Cert shall not be forced as mandatory parameter for saml assertion signing. It is because the dependent lib xml-crypto only requires private key in order to signing xml document, refer https://github.com/yaronn/xml-crypto#signing-xml-documents. Cert is only used to verify the signature of saml response by assertion consumer.

References

#57

Testing

unit test is added to cover the new functionality

• [v] This change adds test coverage for new/changed/fixed functionality

Checklist

• [v] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• [v] All active GitHub checks for tests, formatting, and security are passing
• [v] The correct base branch is being used, if not master