Allow signing with the `cold_pubkey` instead of satisifying the CTV template

[shesek]

This allows to recover funds in case an incorrect amount is sent, and also enables a fast-path to spend out of the vault using the cold key without going through the two-step unvaulting process.

Not going through the unvaulting process does not reduce security because in case of theft these keys will eventually get access to the funds anyway. A thieve could even chain the two-step CTV spend transactions to the cold_key, plus a third transaction to spend from the cold_key to their own wallet, and send them all to the mempool (or straight to a block) in one go.

Here's an example of the suggested changes, expressed in Minsc:

 fn SimpleCtvVault($hot_signer, $cold_signer, $fee_pk, $delay, $amount, $fee) {
   $unvault_amount = $amount - $fee - DUST_AMOUNT;
 
   $tohot = $hot_signer && older($delay);
   $tocold = txtmpl([
     txOut(wsh($cold_signer), $unvault_amount - $fee - DUST_AMOUNT), // - $fee again to pay for the 2nd tx
     txOut(wpkh($fee_pk), DUST_AMOUNT), // fee anchor output
   ]);
 
   txtmpl([
-    txOut(wsh($tocold || $tohot), $unvault_amount),
+    txOut(wsh($tocold || $tohot || $cold_signer), $unvault_amount),
     txOut(wpkh($fee_pk), DUST_AMOUNT), // fee anchor output
-  ])
+  ]) || $cold_signer
 }

(on the min.sc editor)