ni.measurements.* packages and ni.datamonikers.* packages should be x-plat

[nick-beer]

Bug Report

• The README.md at the root of each of these packages lists Windows as the only supported operating system.
• The pyproject.toml only lists Windows as the supported OS

These packages should be windows + posix

Repro or Code Sample

None - see aforementioned files.

Expected Behavior

The packages should support Windows and POSIX

Possible Solution

Update the README.md, update pyproject.toml
Optional: raise an error from __init__ if the grpc channel is not given and we're not on Windows. Other OS do not have DiscoveryClient support (yet), but the exported packages should still claim POSIX/Linux support.

AB#3327918