Add intEnum support for json-schema

[srchase]

This PR adds support for intEnums when converting to json-schema and OpenAPI and closes: #1667. This feature is enabled by default, but can be disabled by setting the disableIntEnums configuration property to true.

Currently, an intEnum is only represented as a "number".

The following Smithy model:

structure Foo {
    bar: TestIntEnum
}

intEnum TestIntEnum {
    FOO = 1
    BAR = 2
}

Is converted as:

{
    "definitions": {
        "Foo": {
            "type": "object",
            "properties": {
                "bar": {
                    "type": "number"
                }
            }
        }
    }
}

This PR adds the enum property to the output, and updates the ref strategy so that intEnums use a $ref the same way as enum shapes.

{
    "definitions": {
        "Foo": {
            "type": "object",
            "properties": {
                "bar": {
                    "$ref": "#/definitions/TestIntEnum"
                }
            }
        },
        "TestIntEnum": {
            "type": "number",
            "enum": [
                1,
                2
            ]
        }
    }
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.