Add Cirrus-CI config file to build & test on FreeBSD

[emaste]

Test on FreeBSD 12.4 and FreeBSD 13.2, the two currently supported releases.

Description

Add CI for FreeBSD using Cirrus-CI Example run: https://cirrus-ci.com/task/6708320812138496

Checklist

• [x] I have read followed CONTRIBUTING.md
  • [ ] I have added tests
  • [ ] I have updated the documentation
  • [ ] I have updated the CHANGELOG
• [ ] Are there any breaking changes?
  • [ ] If yes: I have marked them in the CHANGELOG (example)
• [ ] Does this PR introduce any platform specific code?
• [ ] Security: Does this PR potentially affect security?
• [ ] Performance: Does this PR potentially affect performance?

I did not see a changelog entry for CircleCI, but if this PR is welcome and a changelog entry is desired I will push an update.

[PJK]

Very cool, thank you!

[PJK]

I enabled the GH integration too: https://cirrus-ci.com/github/PJK/libcbor/master