graph/db: add FundingPKScript method on ChannelEdgeInfo

Add FundingPKScript() method that returns the funding output's
pkScript for the channel. The implementation is version-aware:

- V1: generates a 2-of-2 multisig P2WSH script from the two bitcoin
  keys
- V2: will use taproot script (to be implemented)

This encapsulates the script generation logic and makes it clear
which bitcoin keys are being used. Replaces direct calls to
genMultiSigP2WSH with the cleaner method call.

graph/db/sql_store.go

@@ -2768,6 +2768,7 @@ func (s *SQLStore) ChannelView() ([]EdgePoint, error) {
 		handleChannel := func(_ context.Context,
 			channel sqlc.ListChannelsPaginatedRow) error {
 
+			// TODO(elle): update to handle V2 channels.
 			pkScript, err := genMultiSigP2WSH(
 				channel.BitcoinKey1, channel.BitcoinKey2,
 			)