Add new hook allow asymmetric encription

[colombinis]

Description

Thi PR adds new hook jwt_auth_token_before_decode to allow the use of asymmetric encryption

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)
• This change requires a documentation update

How has this been tested?

I've used same examples as in the https://github.com/firebase/php-jwt readme

Steps I've foollowed to test it

• add constant in the wp-config.php
• add hooks in functions.php (theme twentytwentyfour)
• used postman /wp-json/jwt-auth/v1/token , to get the token
• used postman /wp-json/jwt-auth/v1/token/validate , to validate the token

Test Configuration:

• WordPress version: 6.6.2
• PHP version: 7.4

Checklist:

• My code follows the style guidelines of this project (WordPress code standards)
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if needed)
• My changes generate no new warnings
• I have described how I made my tests that prove my fix is effective or that my feature works