Add alias for obtaining the public key

[lbalmaceda]

Description

Attempt to provide backwards compatibility for obtaining the PEM representation of the public key (RSA or not).

When this is merged, users of the library can quickly access the public key instance of the jwk object by calling getPublicKey().

// before
const signingKey = key.publicKey || key.rsaPublicKey;

// now
const signingKey = key.getPublicKey()

References

Nothing linked here yet

Testing

Added unit tests that assert that the pem property has the same value as the publicKey and rsaPublicKey properties.

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

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

[lbalmaceda]

Added readme changes. Will add a small paragraph to the changelog on the next release instructing to use the new method.