Merge bitcoin/bitcoin#20867: Support up to 20 keys for multisig under Segwit context

ebd4be43cc doc: add release notes for 20867 (Antoine Poinsot)
5aa50ab9cc rpc/util: multisig: only check redeemScript size is <= 520 for P2SH (Antoine Poinsot)
063df9e897 test/functional: standardness sanity checks for P2(W)SH multisig (Antoine Poinsot)
ae0429d3af script: allow up to 20 keys in wsh() descriptors (Antoine Poinsot)
9fc68faf35 script: match multisigs with up to MAX_PUBKEYS_PER_MULTISIG keys (Antoine Poinsot)

Pull request description:

  As described in https://github.com/bitcoin/bitcoin/issues/20620 multisigs are currently limited to 16 keys in descriptors and RPC helpers, even for P2WSH and P2SH-P2WSH.

  This adds support for multisig with up to 20 keys (which are already standard) for Segwit v0 context for descriptors (`wsh()`, `sh(wsh())`) and RPC helpers.

  Fixes https://github.com/bitcoin/bitcoin/issues/20620

ACKs for top commit:
  meshcollider:
    re-utACK ebd4be43cc
  instagibbs:
    re-ACK ebd4be43cc

Tree-SHA512: 36141f10a8288010d17d5c4fe8d24878bcd4533b88a8aba3a44fa8f74ceb3182d70fee01427e0ab7f53ce7fab46c88c1cd3ac3b18ab8a10bd4a6b8b74ed79e46

test/functional/wallet_importdescriptors.py

@@ -459,6 +459,77 @@ class ImportDescriptorsTest(BitcoinTestFramework):
         assert_equal(tx_signed_2['complete'], True)
         self.nodes[1].sendrawtransaction(tx_signed_2['hex'])
 
+        self.log.info("We can create and use a huge multisig under P2WSH")
+        self.nodes[1].createwallet(wallet_name='wmulti_priv_big', blank=True, descriptors=True)
+        wmulti_priv_big = self.nodes[1].get_wallet_rpc('wmulti_priv_big')
+        xkey = "tprv8ZgxMBicQKsPeZSeYx7VXDDTs3XrTcmZQpRLbAeSQFCQGgKwR4gKpcxHaKdoTNHniv4EPDJNdzA3KxRrrBHcAgth8fU5X4oCndkkxk39iAt/*"
+        xkey_int = "tprv8ZgxMBicQKsPeZSeYx7VXDDTs3XrTcmZQpRLbAeSQFCQGgKwR4gKpcxHaKdoTNHniv4EPDJNdzA3KxRrrBHcAgth8fU5X4oCndkkxk39iAt/1/*"
+        res = wmulti_priv_big.importdescriptors([
+        {
+            "desc": descsum_create(f"wsh(sortedmulti(20,{(xkey + ',') * 19}{xkey}))"),
+            "active": True,
+            "range": 1000,
+            "next_index": 0,
+            "timestamp": "now"
+        },
+        {
+            "desc": descsum_create(f"wsh(sortedmulti(20,{(xkey_int + ',') * 19}{xkey_int}))"),
+            "active": True,
+            "internal": True,
+            "range": 1000,
+            "next_index": 0,
+            "timestamp": "now"
+        }])
+        assert_equal(res[0]['success'], True)
+        assert_equal(res[1]['success'], True)
+
+        addr = wmulti_priv_big.getnewaddress()
+        w0.sendtoaddress(addr, 10)
+        self.nodes[0].generate(1)
+        self.sync_all()
+        # It is standard and would relay.
+        txid = wmulti_priv_big.sendtoaddress(w0.getnewaddress(), 9.999)
+        decoded = wmulti_priv_big.decoderawtransaction(wmulti_priv_big.gettransaction(txid)['hex'])
+        # 20 sigs + dummy + witness script
+        assert_equal(len(decoded['vin'][0]['txinwitness']), 22)
+
+
+        self.log.info("Under P2SH, multisig are standard with up to 15 "
+                      "compressed keys")
+        self.nodes[1].createwallet(wallet_name='multi_priv_big_legacy',
+                                   blank=True, descriptors=True)
+        multi_priv_big = self.nodes[1].get_wallet_rpc('multi_priv_big_legacy')
+        res = multi_priv_big.importdescriptors([
+        {
+            "desc": descsum_create(f"sh(multi(15,{(xkey + ',') * 14}{xkey}))"),
+            "active": True,
+            "range": 1000,
+            "next_index": 0,
+            "timestamp": "now"
+        },
+        {
+            "desc": descsum_create(f"sh(multi(15,{(xkey_int + ',') * 14}{xkey_int}))"),
+            "active": True,
+            "internal": True,
+            "range": 1000,
+            "next_index": 0,
+            "timestamp": "now"
+        }])
+        assert_equal(res[0]['success'], True)
+        assert_equal(res[1]['success'], True)
+
+        addr = multi_priv_big.getnewaddress("", "legacy")
+        w0.sendtoaddress(addr, 10)
+        self.nodes[0].generate(6)
+        self.sync_all()
+        # It is standard and would relay.
+        txid = multi_priv_big.sendtoaddress(w0.getnewaddress(), 10, "", "",
+                                            True)
+        decoded = multi_priv_big.decoderawtransaction(
+            multi_priv_big.gettransaction(txid)['hex']
+        )
+
+
         self.log.info("Combo descriptors cannot be active")
         self.test_importdesc({"desc": descsum_create("combo(tpubDCJtdt5dgJpdhW4MtaVYDhG4T4tF6jcLR1PxL43q9pq1mxvXgMS9Mzw1HnXG15vxUGQJMMSqCQHMTy3F1eW5VkgVroWzchsPD5BUojrcWs8/*)"),
                               "active": True,