use Vonage JWT generator instead of PyJWT for requests

[maxkahan]

Uses the new vonage jwt generator instead of PyJWT. Custom claims can be specified and it can also be used independently of the python sdk to generate jwts for use with vonage apis.

Refactoring, editing the readme, adding additional tests.

I've also added a new test for verify v2.

[codecov-commenter]

Codecov Report

Merging #262 (6f4796d) into main (896f990) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
+ Coverage   97.03%   97.10%   +0.06%     
==========================================
  Files          21       21              
  Lines         809      794      -15     
==========================================
- Hits          785      771      -14     
+ Misses         24       23       -1     

Impacted Files	Coverage Δ
src/vonage/__init__.py	100.00% <100.00%> (ø)
src/vonage/client.py	93.92% <100.00%> (-0.47%)	:arrow_down:

... and 1 file with indirect coverage changes

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