multi: add raw tx hex to GetTransactions response

[Roasbeef]

In this PR we add the raw tx hex to the GetTransactions call. This is useful as it's now possible to obtain the entire transaction to examine things like the tx ins/outs. Additionally, once #3140 is in, callers can use this to locate change outputs for transaction we send to use for CPFP fee bumping.

[wpaulino]

LGTM ⚡️

Would be nice to add a check within one of the lnwallet tests to make sure the raw bytes deserialize into the expected transaction.

[yaslama]

@Roasbeef @wpaulino What do you think about adding the same field in the transactions sent in the stream returned by SubscribeTransactions? There is already a PR about destAddresses (#1933), and I can improve it to add RawTxHex.

[wpaulino]

@yaslama SGTM. It should probably be included in this PR.

[yaslama]

@Roasbeef @wpaulino The field RawTx needs to be filled in unminedTransactionsToDetail:

diff --git a/lnwallet/btcwallet/btcwallet.go b/lnwallet/btcwallet/btcwallet.go
index 522aac6d..8a45623c 100644
--- a/lnwallet/btcwallet/btcwallet.go
+++ b/lnwallet/btcwallet/btcwallet.go
@@ -561,6 +561,7 @@ func unminedTransactionsToDetail(
                Hash:      *summary.Hash,
                TotalFees: int64(summary.Fee),
                Timestamp: summary.Timestamp,
+               RawTx:     summary.Transaction,
        }
 
        balanceDelta, err := extractBalanceDelta(summary, wireTx)