Pass HttpContext for an operation to use

[diesieben07]

Currently we can pass custom headers into an operation using context: { headers }. It would be nice if we could also pass in HttpContext (docs), to allow passing data to any interceptors.
I would propose an API like so:

const httpContext = new HttpContext();
httpContext.set(...);
whatever.mutate({ vars }, { context: { httpContext } }).subscribe(...);