Introduce PublishRuntimeIdentifier

[nagilson]

Changes Examples

dotnet publish --use-current-runtime -p:PublishRuntimeIdentifier=tizen will publish with tizen, not the ucr.
dotnet publish --self-contained -p:PublishRuntimeIdentifier=win-x86 will publish with rid: win-x86
dotnet publish -p:PublishRuntimeIdentifier=win-x86 will publish with win-x86 rid.

In a project file:

<RuntimeIdentifier>win-x86</RuntimeIdentifier>
<PublishRuntimeIdentifier>win-x64</PublishRuntimeIdentifier>

dotnet build -> win-x86 rid.
dotnet publish -> win-x64 rid.

dotnet publish -r win-x86 /p:PublishRuntimeIdentifier=tizen will get rid: win-x86. Not ideal, we can do it the other way but it will have potentially far-reaching consequences.
dotnet publish --property:RuntimeIdentifier=win-x86 /p:PublishRuntimeIdentifier=tizen: gives win-x86 rid.

Summary

• PublishRuntimeIdentifier will override RuntimeIdentifier when publishing unless it is global, like passed in with dotnet publish --r RID /p:PulishRuntimeIdentifier=RID_2
• PublishRuntimeIdentifier will override the RID if UCR is used.
• Same thing where it doesn't work with t:/Publish but we plan to make it work in VS Publish outside of this PR.

The rest is pretty self explanatory...

[nagilson]

@dsplaisted PTAL when you're free

[nagilson]

/azp run (GH is bugged due to rebase)

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[nagilson]

@dsplaisted PTAL. I've made all of the requested changes and a few others where the same comment applied to other parts. Let's merge this and then use this testProject change in the PublishSelfContained PR.

[nagilson]

@dsplaisted Thanks for taking a look, I appreciate your time. I added some comments in the test code. As incorrect comments can be quite insidious, lmk if you see anything wrong.