feat: add hidden overwrite flag to the openapi upload cmd

[jarrodparkes]

🚥 Resolves ISSUE_ID

🧰 Changes

If an automated system runs openapi upload on a pre-existing file, then an overwrite prompt is shown. There is no way to skip this prompt which can lock up automated systems.

This PR adds a flag to skip the prompt.

🧬 QA & Testing

Provide as much information as you can on how to test what you've done.

Case 1 - confirm existing behavior

• Upload a new API spec to readme using openapi upload {spec} --key={key} --slug={slug}
• Try uploading the API spec again to trigger the overwrite prompt

Case 2 - test new behavior

• Upload a new API spec to readme using openapi upload {spec} --key={key} --slug={slug}
• Try uploading the API spec again using openapi upload {spec} --key={key} --slug={slug} --overwrite to skip the overwrite prompt