adds code gen for optional scalars in swift (#6038)

Adds swift test code

Replaces if statments

Adds swift to supported languages for optionals

Moved std::string to auto

Adds nullable scalars support in object api

tests/FlatBuffers.Test.Swift/Tests/FlatBuffers.Test.SwiftTests/XCTestManifests.swift

@@ -54,6 +54,7 @@ extension FlatBuffersTests {
         ("testOffset", testOffset),
         ("testReadCountry", testReadCountry),
         ("testStartTable", testStartTable),
+        ("testWriteOptionalValues", testWriteOptionalValues),
     ]
 }