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/rpc/client.cpp

@@ -60,6 +60,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "getbalance", 1, "minconf" },
     { "getbalance", 2, "include_watchonly" },
     { "getbalance", 3, "avoid_reuse" },
+    { "getblockfrompeer", 1, "nodeid" },
     { "getblockhash", 0, "height" },
     { "waitforblockheight", 0, "height" },
     { "waitforblockheight", 1, "timeout" },