Improve support for OpenAPI enum items that contain documentation

[BDQ]

Today, if I use the following Smithy enum definition:

@documentation("The status of the enrollment request.")
enum EnrollmentStatus {
  @documentation("The enrollment request is pending review.")
  PENDING = "PENDING",
  @documentation("The enrollment request has been approved.")
  APPROVED = "APPROVED",
  @documentation("The enrollment request has been rejected.")
  REJECTED = "REJECTED"
}

The following OpenAPI output will be produced, as can be seen below, the per item documentation lines are lost:

{
    "EnrollmentStatus": {
        "type": "string",
        "description": "The status of the enrollment. Only Stedi can set or update this property.",
        "enum": [
            "PENDING",
            "APPROVED",
            "REJECTED"
        ]
    }
}

Using OAS 3.1.x (and JSON Schema 2020-12) enums can be defined with oneOf along with const to allow documentation to included for each item, for example:

{
	"$schema": "https://json-schema.org/draft/2020-12/schema",
	"type": "object",
	"properties": {
		"status": {
			"type": "string",
			"description": "The status of the enrollment request.",
			"oneOf": [
				{
					"const": "PENDING",
					"description": "The enrollment request is pending."
				},
				{
					"const": "APPROVED",
					"description": "The enrollment request has been approved."
				},
				{
					"const": "REJECTED",
					"description": "The enrollment request has been rejected."
				}
			]
		}
	}
}

It would be great if we could have the same support in the openapi plugin.