feat: support const for OAS 3.1.1

[markrzen]

Description

Support const as described in OAS 3.1.1.

Resolves: #1142

Motivation and Context

const allows you to declare a constant value.

It also helps compose annotated enumerations, as an improvement to the original enum.

REL: https://swagger.io/specification/#annotated-enumerations

How Has This Been Tested?

I added a test schema (as seen in PR), and I ran it locally. I did not see any unit/integration test patterns in the repository.

Screenshots (if appropriate)

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.

[github-actions]

Visit the preview URL for this PR (updated for commit 9361774):

https://docusaurus-openapi-36b86--pr1143-odq0fqbd.web.app

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[markrzen]

The preview looks good to me.

[markrzen]

@sserrata As the last contributor to the repository, what is the expectation I should have for review?

[sserrata]

LGTM, thanks for the contribution!

[markrzen]

@sserrata Thanks so much. Sorry for the explicit poke. Didn't see any expectation or recommendation in the CONTRIBUTING.md.