support response headers

[mnahkies]

It's possible to declare response headers on the response specifications. Add support for these as per specification here: https://spec.openapis.org/oas/v3.0.3#response-object

openapi-code-generator/packages/openapi-code-generator/src/core/openapi-types.ts

Lines 114 to 117 in b4e5ef6

	// TODO support response headers
	headers?: {
	[headerName: string]: Header | Reference
	}