Add ALLOW_UNUSED_CONFIG_PATHS flag

[katieclaiborne]

resolves #6080

Description

Added a new ALLOW_UNUSED_CONFIG_PATHS flag, which can be used to raise an error when unused config paths are present.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[cla-bot]

Thanks for your pull request, and welcome to our community! We require contributors to sign our Contributor License Agreement and we don't seem to have your signature on file. Check out this article for more information on why we have a CLA.

In order for us to review and merge your code, please submit the Individual Contributor License Agreement form attached above above. If you have questions about the CLA, or if you believe you've received this message in error, don't hesitate to ping @drewbanin.

CLA has not been signed by users: @katieclaiborne

[katieclaiborne]

cc @felippecaso

[jtcohen6]

Awesome work @katieclaiborne, and thanks for the contribution!

This prompted some solid conversation from the team about what a good general-purpose approach here could look like. I'm going to close this PR and the accompanying issue in favor of a new proposal. More context in #6080 (comment)