Define procedure to convert options object of sw.register() to Link header

[surma]

With the recent comment in #893, we have to keep in mind that all the options that navigator.serviceWorker.register accepts need to be available to the Link header (and the <link rel="serviceworker">) as well. With foreign fetch, this becomes especially important.

It would probably be good to define a procedure to convert the options object to a string-based representation. Additionally, we should limit the types of options that can be added in the future.

[marcoscaceres]

It be great for web manifest integration if it's either a string sequence ("options": "...") or something that is JSON friendly {"options": {...}}.

[marcoscaceres]

(of course, web manifest can just use the object form... just haven't spec'ed it, so waiting to see if something nice comes out of this)

[jakearchibald]

Is this still useful now we've removed foreign fetch? What would you use this API for?

[mkruisselbrink]

We haven't removed link headers yet, but if we remove that we probably won't need this, yes.