Add a way to produce "file" style Avro output

[cowtowncoder]

(moved from FasterXML/jackson-dataformat-avro#42 by @srini-daruna)

I created avro schema from class and got the byte array and want to write the data to avro file.
How can i do that.?

[cowtowncoder]

(copied my original comment from source issue)

For better or worse, Avro dictates different format for files than network transport -- Avro files are supposed to have both prefix and pre-written schema as JSON. Currently this module only does "raw" encoded content, and does not have means to add wrapper stuff.
Addition of header and avro schema would be a good addition I think.

Note, however, than when transferring Avro content over the wire, these are not supposed to be used. I don't know if spark (et al) would even allow use; perhaps they try to auto-detect which mode is used.

[cowtowncoder]

Implementation added:

AvroGenerator.Feature.AVRO_FILE_OUTPUT

enabling of which will then prepend schema as expected by Avro for "file" format (with header, schema and then payload), instead of "raw" (rfc) format.