Merge #16240: JSONRPCRequest-aware RPCHelpMan

b6fb617aaaad5f9cdd7f2ad2825b253ca792055d rpc: switch to using RPCHelpMan.Check() (Karl-Johan Alm)
c7a9fc234f3ce400ce78b9b434d2d210b2646c50 Make the RPCHelpMan aware of JSONRPCRequest and add Check() helper (Karl-Johan Alm)
5c5e32bbe3dfa790dd8bb421fbd6301ae10b09f5 rpc: migrate JSONRPCRequest functionality into request.cpp (Karl-Johan Alm)
0ab8ba1ac65b70f044a5e323b13d098cef33695a rpc: fix RPC help requirements for getblocktemplate (Karl-Johan Alm)

Pull request description:

  Every single RPC call has a helper-section at the start, which throws a help string if the user asks for help or if the user provided too few/many arguments.

  ```C++
  const RPCHelpMan help{...};
  if (request.fHelp || !help.IsValidNumArgs(request.params.size())) {
      throw std::runtime_error(help.ToString());
  }
  ```

  or (older version)

  ```C++
  if (request.fHelp || request.params.size() < min || request.params.size() > max)
      throw std::runtime_error(
          RPCHelpMan{...}.ToString()
      );
  ```

  It seems like an obvious improvement, and less copy-pasting, to make `RPCHelpMan` aware of `JSONRPCRequest`, and to let it handle the checks instead. Both of the above become

  ```C++
  RPCHelpMan{...}.Check(request);
  ```

  which means we save roughly 3 lines per RPC command, and the `RPCHelpMan` instance is never referenced afterwards, so the approach is a tiny fraction cleaner.

  This is a complete update, sans a few special case locations that had special rules. 623 lines turn into 284 (which includes the addition to `RPCHelpMan`).

ACKs for top commit:
  laanwj:
    code rview and lightly tested ACK b6fb617aaaad5f9cdd7f2ad2825b253ca792055d
  MarcoFalke:
    ACK b6fb617aaa, looked at the diff, verified move-only where applicable

Tree-SHA512: eb73f47f812512905b852e313281d1c8df803db40a6188aa39d5a7586631664db6764491152a8a96769946c796dc56d38c6e3a66ddd06ba3fb9d20050e6274e1

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