Support setting cookie domain on a per-request basis.

[rahulgi]

This is a proof-of-concept to enable passing a function for the domain config option to dynamically modify the cookie domain based on the incoming request.

It does so by adding a call to a new modifyOptsForRequest helper anywhere cookies.set is called.

modifyOptsForRequest creates a copy of opts, checks whether the opts.domain value is a function, and replaces the domain value with the result of that function if so.

An example of how this might be used:

app.use(session({
  domain: (ctx, opts) => {
    return ctx.get("origin") || DEFAULT_ORIGIN;
  }
});

This specifically addresses https://github.com/koajs/session/issues/188, but it should be easy to extend this PR to support the other cookie config options (like httpOnly, maxAge, etc.). If this approach looks good and maintainers would be interested in the fix, I'd be happy to update this PR to properly do so - let me know!