Refactors out common Checksum concept

[ryanmoran]

Summary

We have some common code for splitting and comparing checksums across cargo and postal. This factors those pieces out into a common place.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[sophiewigmore]

I can't comment on it directly for some reason, but there is a linting error in postal/service.go to remove function determineChecksum

[sophiewigmore]

Also, this seems like a reasonable change but I wanted to point out that introducing the new Checksum type will be breaking for jam, since it uses the cargo.ConfigMetadataDependency internally

[fg-j]

A couple nitpicks/clarifications. But overall, LGTM!

[ryanmoran]

I can't comment on it directly for some reason, but there is a linting error in postal/service.go to remove function determineChecksum

Good catch. I meant to remove this.

[ryanmoran]

Also, this seems like a reasonable change but I wanted to point out that introducing the new Checksum type will be breaking for jam, since it uses the cargo.ConfigMetadataDependency internally

Ok. I rolled back the changes that might have been breaking. We still use Checksum internally and users can cast their checksum string to a Checksum to use its helper methods, so I still think this results in a cleaner implementation.