This is an implementation of https://github.com/lightningnetwork/lightning-rfc/blob/route-blinding/proposals/route-blinding.md (only with --enable-experimental-features).

In this method, the vendor provides a blinded path similar to route hints, but the node ids are blinded, and instead of short_channel_ids for each hop, it provides an encrypted blob. This can be used for (finally, genuinely!) private channels as well as fully private vendor paths. Every node on the blinded path needs to be upgraded, however.

This is the minimum implementation to allow interop testing (e.g. with @t-bast when he gets Eclair done). There are five things remaining to be done:

1. We need to advertize a feature bit (simple to fix)
2. We currently use a dummy short_channel_id field, which is unnecessary in blinded paths (a bit more complex, since our internals assume a short_channel_id field in every hop).
3. We don't propagate errors properly (we need to decide how we'll do this).
4. We haven't yet defined a bolt11 encoding for blinded paths.
5. We don't validate padded entries in the blinding (we could add them, but we need to check they're valid, otherwise the payer can corrupt them and discover if it was a dummy entry).

It's probably premature to add those until we have a solid spec (esp. how to encode blinded routes in bolt11; presumably this will involve emojis!).

Changelog-None: Experimental only