NLog Target support ConnectionString with isolated TelemetryClient

[snakefoot]

Fix Issue #2714 - Add support for connection string to Microsoft.ApplicationInsights.NLogTarget

Changes

• Added ConnectionString option for NLog Target with isolated TelemetryClient (instead of modifying global config)
• Changed NLog Target Flush to use FlushAsync (instead of blocking)

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

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Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

[snakefoot]

@cijothomas + @rajkumar-rangaraj + @TimothyMothra Fork of #2858 where I have changed to use isolated TelemetryConfiguration so not modifying the global config, when using ConnectionString.

I don't mind also fixing log4net-appender if you like. Thus moving away from using InstrumentationKey before it is fully deprecated.

Please review and merge.