Verify manifest checksums match file contents

[djjuhasz]

Fixes #40.

• Add manifest package with a Files() function that stream parses an SFA XML manifest and returns a map of filepaths to checksums
• Switch to manifest.Files() to parse SFA manifest files more efficiently
• Add checksum verification to the "verify manifest" activity
• Move "internal/workflow/testdata/little-Test-AIP-Digitization" to "internal/testdata/little-Test-AIP-Digitization"
• Update "tesdata/little-Test-AIP-Digitization" file hashes to match file contents
• Add a verify checksums event to the preprocessing workflow

[djjuhasz]

Screenshot of the checksum verification failing:

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.68254% with 13 lines in your changes missing coverage. Please review.

Project coverage is 51.40%. Comparing base (06bc9b1) to head (2ef7ef5). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/activities/verify_manifest.go	82.25%	7 Missing and 4 partials :warning:
internal/manifest/manifest.go	95.65%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #54 +/- ## ========================================== + Coverage 48.91% 51.40% +2.49% ========================================== Files 26 27 +1 Lines 1468 1564 +96 ========================================== + Hits 718 804 +86 - Misses 690 696 +6 - Partials 60 64 +4 ```

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