Add option to deconflict errors with same status code for OpenAPI

[srchase]

This PR closes #1649. See also: #1265.

It re-introduces a subset of the changes that were added previously in #1304, but later reverted in #1334. The docs changes from #1304 were not entirely reverted, so those are modified here to reflect the new changes, which include only allowing for the oneOf option to deconflict errors. Contrary to the docs for the previous PR, oneOf is supported by API Gateway when the oneOf is included within a component schema and referenced via a $ref. Using oneOf within the path content directly is not allowed.

Deconflicting errors is opt-in when converting to OpenAPI by setting the following in smithy-build.json:

{
    "plugins": {
        "openapi": {
            "service": "smithy.example#Weather",
            "onErrorStatusConflict": "oneOf"
        }
    }
}

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