Support fetchpriority in preload*Assets

[jfsiii]

If a links function uses fetchpriority: 'low', it doesn't show up in the Link header when using preloadRouteAssets.

i.e. it returns

 Link: </assets/lato-regular-CLXx5VlB.woff2>; rel=preload; as=font;

and not

 Link: </assets/lato-regular-CLXx5VlB.woff2>; rel=preload; as=font; fetchpriority=low

However, fetchpriority is a supported attribute in the Link header; See this example for a stylesheet from the MDN docs

Link: </style.css>; rel=preload; as=style; fetchpriority="high"

It's an important property for our site. The default priority for fonts is highest, but we use font-display: optional; for the custom fonts, so we don't need/want the font files getting downloaded before JS and other more important assets. So we use fetchpriority: low

Looking at the code, it appears it doesn't support pagination via rel either, like

Link: <https://api.example.com/issues?page=2>; rel="prev", <https://api.example.com/issues?page=4>; rel="next", <https://api.example.com/issues?page=10>; rel="last", <https://api.example.com/issues?page=1>; rel="first"

At a minimum, I think the documentation should be updated to detail the constraints/choices. Ideally, it would support all the valid aspects from the spec.