Use firebase JWT

[SecondeJK]

This PR replaces the JWT library with the Firebase JWT PHP Library.

Description

JWTs are now created with Firebase JWT PHP library by default, and use the new SHA256 algorithm instead of the older T1 tokens used by TokBox.

Motivation and Context

Enhanced security.

Types of changes

• 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 to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.