Add Support for An Open API Specification to be Associated with an Entity

[brianhyder]

Context:
The pull request is intended to allow users of the TF provider to have a resource that will allow for associating an Open API Specification (OAS) with a given entity outside of the existing "links" functionality provided with the catalog_entity resource.

The reasoning:

1. In some cases, OAS documents are templated and only the fully rendered version is available build or deploy time and is not committed back to the repository. Therefore, the links functionality does not work well.
2. Separating the ability to associate the OAS back to an entity as a separate resource allows for companies to separate the responsibility of setting up the entity and the creation of the OAS document. In our case, the entity creation is handled by our dev-ops teams while the OAS file is created by the engineering team responsible for the entity/service.

• Unit tests created & ran
• Documentation generated
• Manual testing performed locally with the provider against a cortex instance
• Successful run of acceptance test make target. I can't run them against our 1 instance. I was hoping you all could run them against your test instance and provide feedback.

[shawnburke]

yep looks good thanks!