Add support for storing in-toto Attestations outside the log. (#321)

This adds an "Attestation" method to the entry interface. Entries can
return an attestation that they would like to store.

The attestations are currently stored in GCS, but it supports any blob store.
The feature is turned off with a flag, and we can set a max size as well.

Signed-off-by: Dan Lorenc <dlorenc@google.com>