Merge bitcoin/bitcoin#29277: RPC: access RPC arguments by name

30a6c99935 rpc: access some args by name (stickies-v)
bbb31269bf rpc: add named arg helper (stickies-v)
13525e0c24 rpc: add arg helper unit test (stickies-v)

Pull request description:

  Adds string overloads for the `RPCHelpMan::Arg` and `RPCHelpMan::MaybeArg` helpers to be able to access RPC arguments by name instead of index number. Especially in RPCs with a large number of parameters, this can be quite helpful.

  Example usage:
  ```cpp
  const auto action{self.Arg<std::string>("action")};
  ```

  Most of the LoC is adding test coverage and documentation updates. No behaviour change.

  An alternative approach to #27788 with significantly less overhaul.

ACKs for top commit:
  fjahr:
    Code review ACK 30a6c99935
  maflcko:
    ACK 30a6c99935 🥑
  ryanofsky:
    Code review ACK 30a6c99935. Nice change! Implementation is surprisingly simple and additional unit test coverage is welcome, too.

Tree-SHA512: 4904f5f914fe1d421d32f60edb7c5a028c8ea0f140a2f207a106b4752d441164e073066a6bf2e17693f859fe847815a96609d3cf521e0ac4178d8cd09362ea3d

src/rpc/util.cpp

@@ -24,6 +24,8 @@
 #include <util/string.h>
 #include <util/translation.h>
 
+#include <algorithm>
+#include <iterator>
 #include <string_view>
 #include <tuple>
 
@@ -729,6 +731,16 @@ std::vector<std::pair<std::string, bool>> RPCHelpMan::GetArgNames() const
     return ret;
 }
 
+size_t RPCHelpMan::GetParamIndex(std::string_view key) const
+{
+    auto it{std::find_if(
+        m_args.begin(), m_args.end(), [&key](const auto& arg) { return arg.GetName() == key;}
+    )};
+
+    CHECK_NONFATAL(it != m_args.end());  // TODO: ideally this is checked at compile time
+    return std::distance(m_args.begin(), it);
+}
+
 std::string RPCHelpMan::ToString() const
 {
     std::string ret;