use rsa.PSSSaltLengthEqualsHash to match rfc

[partkyle]

Based on RFC 3447, and other discussions around RSAPSS: the salt length should match the size of the algorithm used for signing the token.

This change will cause existing implementations of the go-jose library to generate new tokens with a salt length matching the chosen hash library, but still allow it to detect the hash size in existing tokens.

Here is a link to another go library with a similar library: dgrijalva/jwt-go#285

Here is a reference to another go library
See a python implementation for a similar example: https://github.com/jpadilla/pyjwt/blob/d25c92ca5e9980ca7bc8b31420bf36e3f4a9e3f0/jwt/algorithms.py#L385

[CLAassistant]

All committers have signed the CLA.

[csstaub]

Looks good to me. Thank you for your contribution @partkyle!