Merge bitcoin/bitcoin#20295: rpc: getblockfrompeer

dce8c4c381 rpc: getblockfrompeer (Sjors Provoost)
b884ababc2 rpc: move Ensure* helpers to server_util.h (Sjors Provoost)

Pull request description:

  This adds an RPC method to fetch a block directly from a peer. This can used to fetch stale blocks with lower proof of work that are normally ignored by the node (`headers-only` in `getchaintips`).

  Usage:
  ```
  bitcoin-cli getblockfrompeer HASH peer_n
  ```

  Closes #20155

  Limitations:
  * you have to specify which peer to fetch the block from
  * the node must already have the header

ACKs for top commit:
  jnewbery:
    ACK dce8c4c381
  fjahr:
     re-ACK dce8c4c381

Tree-SHA512: 843ba2b7a308f640770d624d0aa3265fdc5c6ea48e8db32269b96a082b7420f7953d1d8d1ef2e6529392c7172dded9d15639fbc9c24e7bfa5cfb79e13a5498c8

src/validation.cpp

@@ -3407,6 +3407,7 @@ bool CChainState::AcceptBlock(const std::shared_ptr<const CBlock>& pblock, Block
     // This requires some new chain data structure to efficiently look up if a
     // block is in a chain leading to a candidate for best tip, despite not
     // being such a candidate itself.
+    // Note that this would break the getblockfrompeer RPC
 
     // TODO: deal better with return value and error conditions for duplicate
     // and unrequested blocks.