Handle Ion clob serde behavior properly

[guyilin-amazon]

Current IonClobSerilaizer implementation:

public override string Deserialize(IIonReader reader)
{
    byte[] clob = new byte[reader.GetLobByteSize()];
    reader.GetBytes(clob);
    return Encoding.UTF8.GetString(clob);
}

public override void Serialize(IIonWriter writer, string item)
{
    writer.WriteClob(Encoding.UTF8.GetBytes(item));
}

Problems in current implementation:

1. The Ion clob specification allows arbitrary interpretation of the raw binary data but current implementation only allows UTF-8 encoding, which seems weird because string in C# actually uses UTF-16 encoding.
2. The C# string type will map to Ion string by default. So there is no use case for the Serialize(IIonWriter writer, string item) method to serialize a C# string to Ion clob, only the Deserialize method will be used when deserializing Ion clob data.

We should consider:

1. Allow encoding format to be passed in for Ion clob serde
2. What's the proper mapping between Ion clob type and C# type, probably the clob should map to C# byte array byte[] instead of the current string type.