C++: Add clear() method to SharedString (#8797)

A convenience method to reset to an empty string, like already known
from std::string.

ChangeLog: [C++] Added `SharedString::clear()`

api/cpp/tests/datastructures.cpp

@@ -53,6 +53,14 @@ SCENARIO("SharedString API")
         REQUIRE(str.size() == 5);
     }
 
+    SECTION("clear")
+    {
+        str = "Hello";
+        str.clear();
+        REQUIRE(str.size() == 0);
+        REQUIRE(std::string_view(str.data()) == "");
+    }
+
     SECTION("to_lowercase")
     {
         str = "Hello";