APP-6696 include os_version tag on GOOS=darwin

[abe-winter]

What changed

• add readDarwinTags which supplies os_version platform tag. (on linux systems we already get this from /etc/os-release)
• incidental: make the 5-second timer apply to the entire read tags function, rather than just the readGPUTags function

Why

Macos version targeting for binaries.

Manual testing

On a device with:

$ sw_vers --productVersion
14.6.1

RDK logged:

{"tags":"os_version:14"}

[cheukt]

LGTM, any reason why we don't just return the full version string?

[abe-winter]

why we don't just return the full version string

use case for this is to deploy modules based on compatibility; and today we only have string matching, not semver

(i.e. a module targeting os_version:14 won't match os_version:14.6.1)

if we returned full 14.6.1, module authors would have to build their module for each patch version