Reintroduce phpstan, add baseline

[SecondeJK]

This PR adds the basic configuration for PHPStan and the baseline file.

Description

Baseline is set to 0, the config specifies that we only want PHPStan to work with the src folder (tests do weird and wonderful things that are -supposed- to be bad)

Motivation and Context

Code Quality evolution of the Vonage PHP Library with the next step to run in the pipeline for PRs

How Has This Been Tested?

Tests not relevent

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.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #433 (223cc66) into main (9d8189e) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #433   +/-   ##
=========================================
  Coverage     78.86%   78.86%           
  Complexity     2322     2322           
=========================================
  Files           218      218           
  Lines          6251     6251           
=========================================
  Hits           4930     4930           
  Misses         1321     1321           

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