Convert OAS property names to valid Terraform identifiers

[austinvalle]

Closes #58

This ensures that OAS property names are converted to a valid Terraform identifier if needed and documents the expectations/caveats in the DESIGN.md. This was a lot easier because of the refactoring done in #51 🙏🏻

Notes

• I was going back and forth on removing hypens vs. replacing them with an underscore. I decided to remove them as an "unsupported" character for now, but mostly just because I wanted to keep the logic under 5 steps 😆
• I added the duplicate attributes caveat to the DESIGN.md, I also plan to add a call to the codegen-spec validate function before serializing to ensure an error is raised for that 👍🏻

[austinvalle]

Chatted offline, for now we're going to introduce the name validation in codegen-spec as it'll be less noise to remove the validation later vs. adding it later

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