[avro] Add support for reading schema from Avro-encoded file

(moved from https://github.com/FasterXML/jackson-dataformat-avro/issues/10)

Avro streams may include embedded schema, and since it should be relatively safe to either auto-detect it; or just configure this to be the default if no schema is specified, we should support this mode.

As to sample data, maybe this project:

https://github.com/miguno/avro-cli-examples

has data we could use for confirming proper usage.

A follow-up feature should probably be that of producing & embedded schema; but that'd be a separate RFE.