EXP-263.  Update Avro specs for KijiInput/OutputSpec.

Match API changes from EXP-232.

Avro spec, Avro conversion code matches the updated KijiInputSpec and
KijiOutputSpec (e.g., uses ColumnRequestInput/Output instead of
ExpressDataRequest).

Add support for serializing the schemaSpec members of ColumnRequestInput
and ColumnRequestOutput introduced in EXP-222.

review: https://review.kiji.org/r/1343
issue: https://jira.kiji.org/EXP-263