Remove SENTRY_DSN from required env vars

[ghukill]

Purpose and background context

For local development, or external users of this application, it does not make sense to require a SENTRY_DSN environment variable. While setting this to 'none' would satisfy the config setup, and not use Sentry, it feels unnecessary.

How this addresses that need:

• Moves SENTRY_DSN to optional env vars in Config class

NOTE: this PR ports the code changes from another PR #584 (thanks @cjwetherington!), which we were unable to merge at the time for unrelated CI/CD reasons.

How can a reviewer manually see the effects of these changes?

1- unset SENTRY_DSN as environment variable

2- The following should show CLI help, where formerly this would throw a required env var error:

pipenv run oai --verbose \
-o /tmp/test.xml \
-h http://example.com \
harvest --help

Includes new or updated dependencies?

NO

Changes expectations for external applications?

NO: SENTRY_DSN is no longer required as an env var, but providing it will still get picked up used

What are the relevant tickets?

• None

Developer

• All new ENV is documented in README
• All new ENV has been added to staging and production environments
• All related Jira tickets are linked in commit message(s)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer(s)

• The commit message is clear and follows our guidelines (not just this PR message)
• There are appropriate tests covering any new functionality
• The provided documentation is sufficient for understanding any new functionality introduced
• Any manual tests have been performed or provided examples verified
• New dependencies are appropriate or there were no changes