feat: allow builder to set ActivityKind

[w-lfchen]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

This pr adds a simple method to the ActivityBuilder that allows setting the ActivityKind field of the underlying ActivityArgs.
The test case testing the builder has been updated to utilize this new method.

Additional Notes/Questions

• The links to the contributor guide and the coc in the pr template don't work
• Should I update the docs somewhere to point out that not all ActivityKind types are supported?
  We could link to here, but I'm not sure where I should put this.
• Should I run cargo update?
  I don't think it'll cause any issues and dependencies should be kept up to date

Related Issues

#38 allowed for serialization of this field, so I think it's only logical to allow it to be set in the builder as well.

[Jake-Shadle]

Thanks!