Detect IPFS hints and display extension install prompt

[lidel]

This task was extracted from "Notes on bringing IPFS to Brave": #819 (comment)
Please read the link above before this one.

ps. this work should happen in ipfs branch

How to onboard IPFS users?

• We don't want to run IPFS node by default.
• Opt-in should be a conscious decision (just like Widevine install or opening a Tor tab are)
• Onboarding should be presented just-in-time

IPFS support prompt

The initial idea is for Brave to detect IPFS resource or a hint on a website and ask user if support for IPFS should be installed.

• IPFS support would be delivered by IPFS Companion extension
• Explicit opt-in should happen over a flow similar to Widevine (screenshot), but text about IPFS will be more friendly and educational
  • After IPFS Companion install is successful, the tab that triggered it gets reloaded
  • UX around caching negative decision per site could be the same

IPFS hints

CID (content identifier in IPFS) spec is here, it can be validated to remove false-positives.

Good candidates for IPFS hints that could be detected by Brave itself (very light on resources):

• HTTP request for IPFS resource:
  • path starting with /ipfs/<cid> (path gateway)
  • hostname starting with <cid>.ipfs.* (subdomain gateway)
• HTTP response headers including x-ipfs-path
  (best-effort way to detect DNSLinked sites without expensive lookups)
• JS on a website making a getter call for window.ipfs (or window.ipfs.enable() call itself)
  (reliable signal website could opportunistically use native ipfs support, similar to window.ethereum and MetaMask)

Potentially more expensive hint is to check DNS of every visited domain for TXT record indicating presence of DNSLink (removes the need for initial HTTP request to check for x-ipfs-path header)

[bbondy]

Duping to #10220
Thanks for the info above though 👍