Fix event serialization

[n8maninger]

This changes the wallet.Event JSON serialization to match the Go struct. Unfortunately, this will break the UI again @alexfreska.

• Adds the maturityHeight field. The UI should use this to distinguish between received-but-not-spendable payouts (maybe lower the opacity or add a "lock" icon).
• Changes the val field to data to match the rest of walletd.

{
  "id": "h:13271ead1070290c9cbe39ed7a15aa1107a27029479858ee9ab2fcc237ed4aa0",
  "timestamp": "2024-05-01T11:35:03Z",
  "index": {
    "height": 68278,
    "id": "bid:000000009db92b8834aed7809ebe3ff0184e632e8333bba3802de8cd10d2fc86"
  },
  "maturityHeight": 68423,
  "relevant": [
    "addr:11d68c52e67a800d0ca99867e410cfa9c0c3f5bc1726aa90e88f40b7ad0825c91841904dae10"
  ],
  "type": "contract payout",
  "data": {...}
}