Fix #2710 - Return macOS instead of 'Darwin' and return proper version

[aritchie]

Resolves #2710

Enricher will now check if running in a mac environment (runtime as we don't target mac targetframework) and switch "Darin" to "macOS" as well as set the operating system version properly

NOTES:

• The native library was using compiler checks. This was an easier route for the dotnet repo instead of pinvoking.
• Adds a PlatformFact for xunit, so we can skip tests based on platform

[jamescrosswell]

LGTM as soon as all the CI checks are green.

Thanks @aritchie !

[bruno-garcia]

ideally would go on a major bump because technically breaks saved search and tag distribution based on that OS tag. But I'd say unlikely to be a real disruption and the value of the change is worth the risk.