Handling case when Jwk doesn't have 'use' parameter

[manpreet-compro]

#113

Description

Updated the code to handle the case when 'use' parameter in jwk key is not present on is not having any value. As per spec, https://tools.ietf.org/html/rfc7517#section-4.2

Use of the "use" member is OPTIONAL, unless the application requires its presence.

Added a check to check for presence first, if key is present and having non blank value then compare if value is 'sig' as per spec.

References

This PR is submitted in response to Open issue #24

Testing

Updated test cases to include 2 test cases

• value of 'use' is blank (Should pass)
• value of 'use' is 'enc' (Should fail)

Command npm run test can be used to test the implementation

Checklist

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