feat: Twilio Signature Validation

[pmcanseco]

Fixes #94

Implements Twilio Signature Validation, largely based on https://github.com/twilio/twilio-java . Their implementation can be found here and their test cases here. The documentation used for the algorithm for verifying Twilio Signatures was this one.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[eshanholtz]

Looks great! Thanks for the contribution!