Merge bitcoin/bitcoin#29612: rpc: Optimize serialization and enhance metadata of dumptxoutset output

542e13b293 rpc: Enhance metadata of the dumptxoutset output (Fabian Jahr)
4d8e5edbaa assumeutxo: Add documentation on dumptxoutset serialization format (Fabian Jahr)
c14ed7f384 assumeutxo: Add test for changed coin size value (Fabian Jahr)
de95953d87 rpc: Optimize serialization disk space of dumptxoutset (Fabian Jahr)

Pull request description:

  The second attempt at implementing the `dumptxoutset` space optimization as suggested in #25675. Closes #25675.

  This builds on the work done in #26045, addresses open feedback, adds some further improvements (most importantly usage of compact size), documentation, and an additional test.

  The [original snapshot at height 830,000](https://github.com/bitcoin/bitcoin/pull/29551) came in at 10.82 GB. With this change, the same snapshot is 8.94 GB, a reduction of 17.4%.

  This also enhances the metadata of the output file and adds the following data to allow for better error handling and make future upgrades easier:
  - A newly introduced utxo set magic
  - A version number
  - The network magic
  - The block height

ACKs for top commit:
  achow101:
    ACK 542e13b293
  TheCharlatan:
    Re-ACK 542e13b293
  theStack:
    ACK 542e13b293

Tree-SHA512: 0825d30e5c3c364062db3c6cbca4e3c680e6e6d3e259fa70c0c2b2a7020f24a47406a623582040988d5c7745b08649c31110df4c10656aa25f3f27eb35843d99

src/test/validation_chainstatemanager_tests.cpp

@@ -226,10 +226,13 @@ struct SnapshotTestSetup : TestChain100Setup {
                 // A UTXO is missing but count is correct
                 metadata.m_coins_count -= 1;
 
-                COutPoint outpoint;
+                Txid txid;
+                auto_infile >> txid;
+                // coins size
+                (void)ReadCompactSize(auto_infile);
+                // vout index
+                (void)ReadCompactSize(auto_infile);
                 Coin coin;
-
-                auto_infile >> outpoint;
                 auto_infile >> coin;
         }));