utilize IO reader for Generator package

[tylerslaton]

Story

As a developer, I would like the Generator package to accept an IO reader, So that tests and use can be more generic.

Summary

Currently the Generator package driver is expecting a string to process. Instead we should move more to what is outlined by this comment on a previous PR to the repo:

https://github.com/operator-framework/combo/pull/24#discussion_r749581785

We should aim to clean up functionality (of the Generator driver) up a bit while we iterate. See https://github.com/operator-framework/combo/pull/24/files#r749580994 for more information on one approach.

Doing this will help keep the package more generic and improve the validateFile() function to be more accurate.

Acceptance Criteria

• Generator package driver implement in a more generic way utilizing IO reader
• validateFile() logic updated to confirm the length of the file instead of reading for nil.
• Test for the validateFile() function