Merge revault#101: Spend in get_spend_tx and set_spend_tx messages...

509a48a Spend in get_spend_tx and set_spend_tx messages... (Daniela Brozzoni)

Pull request description:

  ...serialized in base64 instead of hex.

  Closes revault#94

ACKs for top commit:
  darosior:
    ACK 509a48a

Tree-SHA512: 0f06af0a55b8d97dcbd5857a727d37648dc2f827a3f772ff5617904e8aa362000f970b4d5c694cc3d3af95e06ec1e3059c9d0dcaa1b432687ab3e0244e470943

messages.md

@@ -323,7 +323,7 @@ set of Unvault.
     "method": "set_spend_tx",
     "params": {
         "deposit_outpoint": ["txid:vout", "txid:vout"],
-        "spend_tx": "hex of Bitcoin-serialized fully-signed spend transaction"
+        "spend_tx": "base64 of Bitcoin-serialized fully-signed spend transaction"
     }
 }
 ```
@@ -366,7 +366,7 @@ return `null`.
 ```json
 {
     "result": {
-        "spend_tx": "hex of Bitcoin-serialized spend tx"
+        "spend_tx": "base64 of Bitcoin-serialized spend tx"
     }
 }
 ```