let extensions show link previews
Created by: sqs
Adds a new extension API sourcegraph.content.registerLinkPreviewProvider
that can be used to annotate links in a GitHub issue (or other comment) with hover and after content. This can be used, e.g., to show preview cards for JIRA issues (not just other GitHub issues), customer names for private CRM links (to track which customers care about an issue), thumbnails of designs on Figma/Zeplin, etc.
This is only implemented for GitHub now, but the hooks in the CodeHost
interface can be used to add support for other code hosts.
Test plan
See test plan for https://github.com/sourcegraph/sourcegraph/pull/3392.
The extension API is marked as experimental for now. It is also behind a feature flag in the browser extension (off by default):