Added support to read diagnostics config from the path defined in environment variable.

[Yun-Ting]

Fix Issue # .

Changes

(Please provide a brief description of the changes here.)

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.