chore: autoload JWT classes

[philnash]

Works towards fixing #396.

Others are reporting that twilio-ruby takes a lot of memory. My belief is that this is because it is a large library that covers an ever increasing API surface. The issue is not necessarily in that the library is so large, but that the entire thing is loaded in one go with requires for days.

I am going to send a few PRs that update our use of require to autoload where appropriate. autoload only loads the file when the constant (class/module) is used.

This is the second PR, after #498, for those who include the library but don't use Access Tokens. The files governing Access Token and JWT creation will not be loaded into memory. For those that do use Access Tokens the behaviour will be unchanged. Notably this also moves requiring jwt to the JWT files too.

This PR will not significantly reduce the memory usage, but it is a further movement towards autoloading the whole library, and smaller PRs are easier to review.

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 master branch
• I have added tests that prove my fix is effective or that my feature works (all existing tests pass)
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[childish-sambino]

LGTM