adding UTF8 String coder to Java SDK ModelCoders

It looks like UTF-8 String Coder in Java and Python SDKs uses different encoding schemes. StringUtf8Coder in Java SDK puts the varint length of the input string before actual data bytes however StrUtf8Coder in Python SDK directly encodes the input string to bytes value. We should unify the encoding schemes of UTF8 strings across the different SDKs and make it a standard coder.