Support attaching URLs to link to other content

## Problem

In Cucumber, attachments work by providing the attachment content to Cucumber which then includes it in the messages.

But sometimes you might just want to _link_ to content that exists elsewhere but is relevant to this test case/step. Maybe it's a dump of logs, or some screenshots, or something else - anything addressable by a URL. Currently there's no way to do this. The schema for attachments has a `url` field, but this is intended for externalising regular attachments as a post-processing step (as done in https://github.com/cucumber/html-formatter/issues/281) rather than linking to already-external content.

## Proposal

We can use the [existing sort-of-standard MIME type](https://www.iana.org/assignments/media-types/text/uri-list) `text/uri-list` for this. The idea is to:

- Provide a `link` function in Cucumber implementations that acts as a shorthand for attaching a URL with the above MIME type
- In formatters - particularly the HTML formatter - render those attachments as a simple link

This is a similar approach to what we've done before with `log`.