Refactor to JWT library

[SecondeJK]

This PR marks the first move to remove the dependency for lcobucci/jwt to vonage/jwt.

Description

While vonage/jwt still uses lcobucci/jwt under the hood, I am extracting it as a dependency for core so that the vonage/jwt library can firstly be reused for other projects that require vonage-specifc JWTs.

Motivation and Context

Preparing core for v5.0 by adjusting dependencies.

How Has This Been Tested?

Existing tests pass.

Example Output or Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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

[codecov-commenter]

Codecov Report

Merging #441 (fdd9215) into main (9ebd564) will increase coverage by 0.04%. The diff coverage is 94.44%.

@@             Coverage Diff              @@
##               main     #441      +/-   ##
============================================
+ Coverage     78.87%   78.92%   +0.04%     
  Complexity     2324     2324              
============================================
  Files           218      218              
  Lines          6253     6248       -5     
============================================
- Hits           4932     4931       -1     
+ Misses         1321     1317       -4     

Files	Coverage Δ
src/Client/Credentials/Keypair.php	91.17% <94.44%> (+9.12%)	:arrow_up:

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