Build public key once on Jwk

[LelvProgrammer]

Changes

This is just a silly change for creating the public key only once for a Jwk. This improvement lies in the fact that the Jwk is designed as an immutable class, but the method for getting the public key generates a new instance all the time out of values that never change. Therefore, it would be better to just store this information once it's built and not have to regenerate it every time it's asked for - which happens often when one considers that Jwk objects tend to be cached.

Testing

No new test was added, as no functionality was added nor has any contract changed - a new private attribute was added for storing some information.

Checklist

• [x ] I have read the Auth0 general contribution guidelines
• [ x] I have read the Auth0 Code of Conduct
• [x ] All existing and new tests complete without errors