message: rpc: Use Join helper in rpc/util
diff: #include <key_io.h>
  #include <outputtype.h>
  -#include <script/signingprovider.h>
  #include <rpc/util.h>
  #include <script/descriptor.h>
  +#include <script/signingprovider.h>
  #include <tinyformat.h>
  #include <util/strencodings.h>
  +#include <util/string.h>
  #include <tuple>
  @@ -645,11 +646,7 @@ std::string RPCArg::ToString(const bool oneline) const
  }
  case Type::OBJ:
  case Type::OBJ_USER_KEYS: {
  - std::string res;
  - for (size_t i = 0; i < m_inner.size();) {
  - res += m_inner[i].ToStringObj(oneline);
  - if (++i < m_inner.size()) res += ",";
  - }
  + const std::string res = Join(m_inner, ",", [&](const RPCArg& i) { return i.ToStringObj(oneline); });
  if (m_type == Type::OBJ) {
  return "{" + res + "}";
  } else {
  