refactor: increase string_view usage

Update select functions that take a const std::string& to take a
std::string_view instead. In a next commit, this allows us to use
the {Arg,MaybeArg}<std::string_view> helper.

src/common/messages.cpp

@@ -16,6 +16,7 @@
 #include <cassert>
 #include <map>
 #include <string>
+#include <string_view>
 #include <utility>
 #include <vector>
 
@@ -91,7 +92,7 @@ std::string InvalidEstimateModeErrorMessage()
     return "Invalid estimate_mode parameter, must be one of: \"" + FeeModes("\", \"") + "\"";
 }
 
-bool FeeModeFromString(const std::string& mode_string, FeeEstimateMode& fee_estimate_mode)
+bool FeeModeFromString(std::string_view mode_string, FeeEstimateMode& fee_estimate_mode)
 {
     auto searchkey = ToUpper(mode_string);
     for (const auto& pair : FeeModeMap()) {