feat: Add Beta OpenAPI v3.1 Spec

[wadethemaker]

feat: add OpenAPI v3.1 spec to repo

This PR adds a Beta version of the an OpenAPI v3.1 specification. It is located in the oas_v3.json file and spec directory.

Documentation outlining how this change and how it affects the repository going forward is added to the README and CONTRIBUTING guide.

This PR is relatively large. I apologize. It represents some cleanup to other files to improve the Twilio SendGrid API reference. We will make smaller PRs that are more easily reviewed going forward.

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.

[thinkingserious]

🎆