Add Nano Server to the list

[mattleibow]

Description of Change

This PR should just create 2 new NativeAssets packages for Microsoft Nano Server.

Because Nano Server is really just Win32 without some features, you actually can just build it using the same code, but with a few extra args - basically a new #define SK_BUILD_FOR_NANOSERVER

.\bootstrapper.ps1 -t externals-windows --buildarch=x64 --additionalGnArgs="extra_cflags+=[''''-DSK_BUILD_FOR_NANOSERVER'''']"

all those extra single quotes (4 of them) are required to make sure powershell doesn't mess up the parameters.

TODO:

• tests
• some sort of verification like with NoDependencies for linux

Bugs Fixed

• Resolves Support Windows Nano Server #676

API Changes

None.

Behavioral Changes

None.

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Updated documentation

[mattleibow]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mattleibow]

Check out this build's artifacts: https://dev.azure.com/xamarin/public/_build/results?buildId=9982&view=results

[mattleibow]

This will be pulled in with #1040.