[nuget] add publish warning when RuntimeIdentifier is not specified

[lachmatt]

Why

Warn when an app uses autoinstrumentation NuGet package, and is published without specifying runtime identifier.
In such scenario, native profiler libraries for all of the supported platforms are copied to the publish output directory, which results in increased disk usage.

Fixes #

What

• Add a MSBuild target that runs before the publish, and issues a warning when runtime identifier is not specified
• Allow the warning to be suppressed by setting DisableAutoInstrumentationCheckForRuntimeIdentifier property

I considered making target run before build instead of publish, and issue an Error instead of a Warning (would be consistent with other target), but decided it would be to noisy/restrictive. Let me know if you think I should change the approach.

Tests

Included in PR.

Checklist

• CHANGELOG.md is updated.
• Documentation is updated.
• New features are covered by tests.

[Kielek]

IMO worth to add changelog entry.