Responses object doesn't support extensions

[TristanSpeakEasy]

According to the OpenAPI Spec the responses object should be able to have extensions added to it https://spec.openapis.org/oas/v3.0.3#responses-object This object MAY be extended with [Specification Extensions](https://spec.openapis.org/oas/v3.0.3#specificationExtensions).

But the latest version of kin-openapi doesn't expose extensions for the openapi3.Responses type which at the moment looks to be just a type alias to a map

[fenollp]

Correct! Responses should be turned into a

struct {
	Extensions map[string]interface{} `json:"-"`

	map[string]*ResponseRef `json:"responses"`
}

with custom Un/MarshalJSON implementations. I'm in the middle of #513 but that work should make the fix for this issue easier.

Also

kin-openapi/openapi3/response.go

Lines 31 to 33 in 46e0df8

	func (responses Responses) Get(status int) *ResponseRef {
	return responses[strconv.FormatInt(int64(status), 10)]
	}

needs to try 21X and 2XX type fields...