chore: Replace Travis-CI with GitHub Actions

[wadethemaker]

test!:

1. Replaces Travis-CI with GitHub Actions.h
2. Replaces Prance validator with Spectral linter.
3. Updates README and CONTRIBUTING. This project is in pre-pilot and should not currently be relied on. This project will be generated by tooling and will not be able to accept public PRs in the near future.

These changes should start to bring the CI/CD pipeline in line with the Twilio OAI repo, which has moved off Travis and onto GitHub Actions.

Spectral lint currently fails. All oai files are being linted, several of which need to have multiple errors addressed.

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.

[wadethemaker]

The last commit sets it to lint only the v3 spec. It's failing but should pass once we pull in PRs to sync this with the other spec work Elmer is doing.

[shwetha-manvinkurke]

Can we make sure the v3 spec lints fine with spectral before this PR is merged?