Allow decentralized component updates to be served

[bbondy]

Currently only extensions that are on the Google store can be installed.

It would be neat to have the ability for user's to set their own component update server URL in Brave. Once this is set, perhaps via a local environment variable. We could allow users of Brave browser to point to our go-update endpoint with a particular subdomain. Perhaps via a location that requires administrator access, like an admin policy.

This particular subdomain could be of the format https://<libp2p-key>.go-updater.brave.com/extensions

When this is done, go-update could merge the served manifest with one controlled by an IPNS URI.

Alternatively, users could host their own go-update servers with their own backend and offering their own extensions. For an extension request that they didn't know about, they'd forward it to our component update server, just like we do when we don't know about an ID to Google's.