Merge bitcoin/bitcoin#28553: validation: assumeutxo params mainnet

1610643 chainparams: add mainnet assumeutxo param at height 840_000 (Sjors Provoost)

Pull request description:

  This adds snapshot parameters for mainnet block 840,000.

  You can generate the snapshot yourself using `./contrib/devtools/utxo_snapshot.sh` or download my torrent:
  * torrent: `magnet:?xt=urn:btih:596c26cc709e213fdfec997183ff67067241440c&dn=utxo-840000.dat&tr=udp%3A%2F%2Ftracker.bitcoin.sprovoost.nl%3A6969`

  It would be a good idea to test:
  1. That you can produce the same snapshot file, sha256 sum:

  ```
  dc4bb43d58d6a25e91eae93eb052d72e3318bd98ec62a5d0c11817cefbba177b utxo-840000.dat
  ```

  2. That the snapshot works

ACKs for top commit:
  fjahr:
    re-ACK 1610643
  achow101:
    ACK 1610643
  theStack:
    Tested ACK 1610643
  mzumsande:
    tested ACK 1610643
  willcl-ark:
    tACK 1610643

Tree-SHA512: 581d8e86379bb044324f04f8559dd0a8946b6e2b145d5f25b38727b30b8cf13d6ac3c8777ff06554d3cf1a072809f7b5fbd693239868578f25dceafe5ba5f57c

src/kernel/chainparams.cpp

@@ -185,7 +185,12 @@ class CMainParams : public CChainParams {
         };
 
         m_assumeutxo_data = {
-            // TODO to be specified in a future patch.
+            {
+                .height = 840'000,
+                .hash_serialized = AssumeutxoHash{uint256{"a2a5521b1b5ab65f67818e5e8eccabb7171a517f9e2382208f77687310768f96"}},
+                .m_chain_tx_count = 991032194,
+                .blockhash = consteval_ctor(uint256{"0000000000000000000320283a032748cef8227873ff4872689bf23f1cda83a5"}),
+            }
         };
 
         chainTxData = ChainTxData{