rpc: add named arg helper

Overload the Arg and MaybeArg helpers to allow accessing arguments
by name as well.

Also update the docs to document Arg and MaybeArg separately

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>
 
@@ -728,6 +730,16 @@ 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;