Generalize build_keys_helper

Generalize build_keys_helper

When constructing a blinded path, two iterators are used: one for the
pubkeys and another for Writeable TLVs. The first iterator is used in
the build_keys_helper utility function while the second is used inside
of a callback. Update this utility to work on any type that can be
borrowed as a PublicKey. This allows for using a single iterator of
tuples, which is necessary for padding the hops without additional
allocations and clones.