elements,pset: populate elements specific data for PSBTs

PSETs have a bit different requirements. The witness_utxo needs
the asset tag + values, and these should also be added to the PSET
struct separately as well. To do this, we create a new 'init' method for
elements inputs, which takes care of the elements specific things.

bitcoin/chainparams.h

@@ -9,6 +9,8 @@
 #include <common/bip32.h>
 #include <stdbool.h>
 
+#define ELEMENTS_ASSET_LEN 33
+
 struct chainparams {
 	const char *network_name;
 	const char *bip173_name;