Co-authored-by: Calvin Zachman <calvin.zachman@protonmail.com>

Add an option to disable route blinding, failing back any HTLC with
a blinding point set when we haven't got the feature enabled.

Note that this commit only handles the case where we're chosen as the
relaying node (where the blinding point is in update_add_htlc), we'll
add handling for the introduction node case once we get to handling of
blinded payloads).

This commit introduces a blinding kits which abstracts over the
operations required to decrypt, deserialize and reconstruct forwarding
data from an encrypted blob of data included for nodes in blinded
routes.

When we have a HTLC that is part of a blinded route, we need to include
the next ephemeral blinding point in UpdateAddHtlc for the next hop. The
way that we handle the addition of this key is the same for introduction
nodes and relaying nodes within the route.

To separate blinded route parsing from payload parsing, we need to
return the parsed types map so that we can properly validate blinded
data payloads against what we saw in the onion.

If we received a payload with a encrypted data point set, our forwarding
information should be set from the information in our encrypted blob.
This behavior is the same for introduction and relying nodes in a
blinded route.

Reject any HTLCs that use us as an introduction point in a blinded
route if we have disabled route blinding. We have to do this after
we've processed the payload, because we only know we're an introduction
point once we've processed the payload itself.

Note: the itest is broken up into multiple commits to make it
more readable, they can be squashed post-review.

We don't support receiving blinded in this PR - just intercept and
settle instead. The HTLC's arrival on the interceptor indicates that
it was successfully forwarded on a blinded hop.

This commit turns off route blinding for the daemon while we're waiting
on full handling for blinded errors. The feature remains on for itests
so that tests covering blinding can run as usual.