adding azure developer credential

[vhvb1989]

• Credential and implementation
• unit test
• readme

Description

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All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-identity

[ghost]

Hello @billwert!

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