[OAS] Create 'colorpalette' OAS spec

QHttpServer has a colorpalette server example, and qtdoc repo has a colorpalette QML application using this server. That application can furthermore use the online https://reqres.in/ service as the server (QHttpServer example implements same API as the reqres.in site)

In addition to using the de facto OpenAPI development specification (petshop), having also a spec for the colorpalette will be useful for development. In the long run we want to provide this as an example application.

Also having the spec available early on in the development allows to see how an already existing QML application can be migrated to using generated code.