feat: add support for string_view as D-Bus type

tests/unittests/Message_test.cpp

@@ -119,7 +119,7 @@ namespace my {
 
 template <>
 struct sdbus::signature_of<my::Struct>
-        : sdbus::signature_of<sdbus::Struct<int, std::string, std::list<double>>>
+    : sdbus::signature_of<sdbus::Struct<int, std::string, std::list<double>>>
 {};
 
 /*-------------------------------------*/
@@ -204,6 +204,21 @@ TEST(AMessage, CanCarryASimpleInteger)
     ASSERT_THAT(dataRead, Eq(dataWritten));
 }
 
+TEST(AMessage, CanCarryAStringAsAStringView)
+{
+    auto msg = sdbus::createPlainMessage();
+
+    const std::string_view dataWritten = "Hello";
+
+    msg << dataWritten;
+    msg.seal();
+
+    std::string dataRead;
+    msg >> dataRead;
+
+    ASSERT_THAT(dataRead, Eq(dataWritten));
+}
+
 TEST(AMessage, CanCarryAUnixFd)
 {
     auto msg = sdbus::createPlainMessage();

tests/unittests/TypeTraits_test.cpp

@@ -85,6 +85,7 @@ namespace
     TYPE(double)HAS_DBUS_TYPE_SIGNATURE("d")
     TYPE(const char*)HAS_DBUS_TYPE_SIGNATURE("s")
     TYPE(std::string)HAS_DBUS_TYPE_SIGNATURE("s")
+    TYPE(std::string_view)HAS_DBUS_TYPE_SIGNATURE("s")
     TYPE(sdbus::BusName)HAS_DBUS_TYPE_SIGNATURE("s")
     TYPE(sdbus::InterfaceName)HAS_DBUS_TYPE_SIGNATURE("s")
     TYPE(sdbus::MemberName)HAS_DBUS_TYPE_SIGNATURE("s")
@@ -122,10 +123,11 @@ namespace
                                 >,
                                 sdbus::Signature,
                                 sdbus::UnixFd,
-                                const char*
+                                const char*,
+                                std::string_view
                             >
                         >;
-    TYPE(ComplexType)HAS_DBUS_TYPE_SIGNATURE("a{t(a{ya(oanbva{is})}ghs)}")
+    TYPE(ComplexType)HAS_DBUS_TYPE_SIGNATURE("a{t(a{ya(oanbva{is})}ghss)}")
 
     typedef ::testing::Types< bool
                             , uint8_t
@@ -138,6 +140,7 @@ namespace
                             , double
                             , const char*
                             , std::string
+                            , std::string_view
                             , sdbus::BusName
                             , sdbus::InterfaceName
                             , sdbus::MemberName