Add an activity to create a BagIt bag (refs #11)

[djjuhasz]

• Upgrade Go version in go.mod to 1.21.4 as required by github.com/nyudlts/go-bagit
• Add a bagit.CreateBagActivity() for creating BagIt bags

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.11765% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 80.92%. Comparing base (c6647df) to head (82579f8).

:exclamation: Current head 82579f8 differs from pull request most recent head 0d1f5de

Please upload reports for the commit 0d1f5de to get more accurate results.

Files	Patch %	Lines
bagit/create_bag_activity.go	90.90%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16 +/- ## ========================================== + Coverage 77.69% 80.92% +3.22% ========================================== Files 3 5 +2 Lines 139 173 +34 ========================================== + Hits 108 140 +32 - Misses 17 18 +1 - Partials 14 15 +1 ```

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[djjuhasz]

@jraddaoui I've added bagit.Config Validate() method that uses https://pkg.go.dev/github.com/go-playground/validator/v10 to do the validation. I think using https://pkg.go.dev/github.com/go-playground/validator/v10 is overkill for validating this one field, but I wanted to explore it as a general validation library we could potentially use for general config validation in Enduro.

Before I tried this validation method I went down the road of defining a HashAlgorithm "enum" type and validating against that. Implementing an enum in Go has it's own drawbacks though (see https://github.com/artefactual-sdps/enduro/issues/516), so I decided to try https://pkg.go.dev/github.com/go-playground/validator/v10.

I'm open to using a validator or an enum here, as there are trade-offs for both methods. What do you think?

[djjuhasz]

P.S. obviously I could just implement a simple Validate() method for "ChecksumAlgorithm" that doesn't use any third-party libraries, but I think this is a general problem that calls for a general solution.