Make TelemetryConfiguration configurable in ApplicationInsightsLoggingBuilderExtensions

[TimothyMothra]

Fix Issue #1944.
Supersedes PR #1643.

Changes

• expose method to allow users to configure the full TelemetlyConfiguration object in ApplicationInsightsLoggingBuilderExtensions. This provides a mechanism to configure the ConnectionString which is now the recommended way to configure Application Insights resources.
• fix link in Readme
• update code sample in Readme to demonstrate setting ConnectionString

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.

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• Design discussion issue #
• Changes in public surface reviewed

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