Add psbt example

Add an example PSBT workflow. The workflow we simulate is that of a
setup using a watch-only online wallet (contains only public keys) and a
cold-storage wallet (contains the private keys).

We create and update a PSBT using the watch-only wallet then pass the
PSBT to the cold-storage wallet to sign.

Co-authored-by: Dan Gould <d@ngould.dev>

Author: tcharding

Cargo.toml

@@ -56,3 +56,7 @@ name = "bip32"
 [[example]]
 name = "handshake"
 required-features = ["std"]
+
+[[example]]
+name = "ecdsa-psbt"
+required-features = ["std", "bitcoinconsensus"]