Allow overriding credentials via rest directive

[stramel]

It would be great to be able to specify an override for credentials via the @rest directive. The majority of my requests need same-origin but I have a handful that need omit specified. Currently, I can't figure out a way to handle this use-case.

[fbartho]

I would write a customFetch implementation then. (It's a thin wrapper around the node-fetch instance).

Alternatively, you can use a custom context link to mess with the headers before they get sent.

[stramel]

@fbartho Would you be willing to accept a PR for this as a feature/enhancement?

[fbartho]

@stramel I keep starting a response to your request, and losing track of the browser tab before submitting.

I wanted to think about it carefully before responding with a reflex gut reaction.

Right now, I feel like the @rest( directive already has too many "optional" arguments with defaults. (Each additional argument is a significant amount of code & testing & documentation & examples -- and each one makes things harder for newbies to know how to use this library). When you asked me about this issue, that's was my immediate thought. Then today, this ticket got opened: #215 and that cemented my feeling.

I really don't think that we should do this as a new optional @rest() parameter. Instead if you implement the customFetch. It would be straight forward for you to create your own fetch with a registry of special overrides:

// Somewhere:
const credentialsRegistryByPathPrefix = {
  "https://example.com/must-omit": { omit: true }
}

// Where you configure RestLink:
function myFetch(path, options) {
  for(const p in credentialsRegistryByPathPrefix) {
    if (path.startsWith(p)) {
      options.credentials = { ...credentialsRegistryByPathPrefix[p], ...(options.credentials || {} ) };
    }
  }
  return fetch(path, options);
}

const restLink = new RestLink({ customFetch: myFetch });

Obviously this is pseudo-code that I just wrote in my browser, but I think it would work.