rpc: refactor: use string_view in Arg/MaybeArg

Modernizes interface by not forcing users to deal with raw pointers,
without adding copying overhead. Generalizes the logic of whether
we return by value or by optional/pointer.

In cases where functions take a `const std::string&` and it would
be too much work to update them, a string copy is made (which was
already happening anyway).

src/wallet/test/wallet_rpc_tests.cpp

@@ -17,7 +17,7 @@ static std::string TestWalletName(const std::string& endpoint, std::optional<std
 {
     JSONRPCRequest req;
     req.URI = endpoint;
-    return EnsureUniqueWalletName(req, parameter ? &*parameter : nullptr);
+    return EnsureUniqueWalletName(req, parameter);
 }
 
 BOOST_FIXTURE_TEST_SUITE(wallet_rpc_tests, BasicTestingSetup)