fix: move require into TwilioWebhookAuthentication initialize (Fixes twilio/twilio-ruby#592)

[engshien]

Fixes

Fixes #592

I have moved the require into the initialize so it is only executed if the user needs the functionality. This is an alternative fix to just requiring the rack gem in the gemspec.

I have run the existing tests and my own tests that were failing in 5.63.1 due to the issue.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] 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

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[philnash]

Thank you for your contribution! Since this middleware is only important when rack is part of the application we've changed this in #602 so that if Rack is not present, the middleware doesn't get loaded. Version 5.65.1 was just released with this update, so you can now install the latest version and it will work again.