Add option to move files to processed directory after submission

[weslambert]

Describe the change
Allows files to be moved to another directory (out of staging) after being submitted for scanning. This directory can be specified by adding files.processed to filestream.yaml.

Describe testing procedures
Tested with:

• /nsm/strelka/unprocessed as files.patterns
• /nsm/strelka/staging as staging
• /nsm/strelka/processed as files.processed

1. echo "MyTestFileContent" > /nsm/strelka/unprocessed/mytestfile
2. File is moved by Strelka into /nsm/strelka/staging.
3. File is submitted for scanning, then moved to /nsm/strelka/processed.

Shout-out to @rwwiv for his help with this, bc I can't Golang good 😆 .

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[phutelmyer]

Gave this a few tests, and it looks good to me.
Thanks @weslambert and @rwwiv!