Added extension support

[RangerMauve]

Following up on the hypercore extension support, this surfaces the extensions API in hyperdrive so that applications can build on top of it.

This could simplify code for things like Beaker's datPeers API.

Adds:

• extensions option in constructor that gets passed to the metadata hypercore
• extension method for sending an extension message to all peers

TODO:

• [x] Tests
• [x] Docs

Should I maybe redo this PR against the v10 branch, or could we get it into a v9 release before then?

[RangerMauve]

This is ready for review. Just need to wait for hypercore 7.5.0 to be released on NPM, then I'll need to update the package.json version