DEVX-7707: Add Webhook Token Verification Functionality

[superchilled]

This PR adds functionality to the SDK to verify JWTs included in Vonage webhooks. Specifically it:

• Updates the gemspec dependency for vonage-jwt to the new package
• Adds an SDK-specific verify_hs256_signature method to the Vonage::JWT class
• Adds verify_webhook_token wrapper methods to Voice and Messaging
• Adds a verify_webhook_sig wrapper method to SMS
• Adds tests for the new functionality
• Updates the README to explain the new functionality

[superchilled]

WIP. Do not merge.

Ready to merge

[codecov-commenter]

Codecov Report

Merging #291 (2f2b46b) into main (e456fd3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 2f2b46b differs from pull request most recent head 34e8f9e. Consider uploading reports for the commit 34e8f9e to get more accurate results

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
+ Coverage   97.93%   97.94%   +0.01%     
==========================================
  Files         108      108              
  Lines        1988     1999      +11     
==========================================
+ Hits         1947     1958      +11     
  Misses         41       41              

Files	Coverage Δ
lib/vonage/jwt.rb	100.00% <100.00%> (ø)
lib/vonage/messaging.rb	100.00% <100.00%> (ø)
lib/vonage/signature.rb	100.00% <100.00%> (ø)
lib/vonage/sms.rb	100.00% <100.00%> (ø)
lib/vonage/voice.rb	100.00% <100.00%> (ø)

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