Handle BOM and other encoding in url request

[CedricGuillemet]

For Android:

• BabylonNative/Dependencies/UrlLib/Source/Android/UrlRequest.cpp

  Line 74 in 1b40d7b

  LoadAsset(assetsManager, path.data(), m_responseString);

• BabylonNative/Dependencies/UrlLib/Source/Android/UrlRequest.cpp

  Line 108 in 1b40d7b

  m_responseString = byteArrayOutputStream.ToString("UTF-8");

For Apple:

• BabylonNative/Dependencies/UrlLib/Source/Apple/UrlRequest.mm

  Line 76 in 1b40d7b

  m_responseString = std::string{static_cast<const char*>(data.bytes), data.length};

UWP appears to be okay for BOM but still should do something about content-type.

TODO:

• Handle BOM correctly for Android / Apple
• Investigate handling of content-type for HTTP
• Investigate file encoding on Win32. See UrlRequest_Win32.cpp:68-85.