Add support for GenericFixed to AzureBlobAvroWriter

Add support for avro type GenericFixed to AzureBlobAvroWriter.write(). This will allow the caller to provide both Schema and an encoded message when sending messages to the SystemProducer. This types avoids the message encoding required by IndexedRecord and still provides the Schema to the writer instance.

The existing AzureBlobAvroWriter allows either an IndexedRecord or a byte[], but requires that the first message is always an IndexedRecord. The existing writer uses the first message to "set" the Schema, }}subsequent messages can be {{{}byte[]s. This requires the sender be aware of the writer's internal state and how it's state is changed by flush() and close(). The writer can use the GenericFixed to "set" the Schema since the Schema is provided by the common parent type GenericContainer. 

Avro reference:

• GenericFixed
• GenericContainer