Added http proxy support for guest agent.

[sanmishra18]

---

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• [ ] Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• [ ] Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• [ ] My extension description/summary conforms to the Extension Summary Guidelines.

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Once your PR is merged into master branch, a new PR will be created to update src/index.json automatically.
The precondition is to put your code inside this repo and upgrade the version in the PR but do not modify src/index.json.

[yonzhan]

connectedvmware

[zhoxing-ms]

Could you add some test cases for this new parameter?

[zhoxing-ms]

@sanmishra18 Any update?