Add ArchiveSize and FileSize to parser records

[stephen-soltesz]

This change adds two new fields to the ParseInfo record for all datatypes:

• ArchiveSize - The original archive size as found in GCS.
• FileSize - The size of the file data provided to the parser.

To populate the ArchiveSize field, this change adds GetSize() to the TestSource interface and GCSSource implementation.

Part of:

• https://github.com/m-lab/etl/issues/1116

---

This change is 

[stephen-soltesz]

Uh oh -- we're doin' it!

Thank you!