Add onion messages module + enable the construction of blinded routes

Add onion messages module + enable the construction of blinded routes

Blinded routes can be provided as destinations for onion messages, when the
recipient prefers to remain anonymous.

We also add supporting utilities for constructing blinded path keys, and
control TLVs structs representing blinded payloads prior to being
encoded/encrypted. These utilities and struct will be re-used in upcoming
commits for sending and receiving/forwarding onion messages.

Finally, add utilities for reading the padding from an onion message's
encrypted TLVs without an intermediate Vec.