[.Net] Support raw-data in ImageMessage

[DavidLuong98]

Why are these changes needed?

This PR supports data url for ImageMessage which will be sent to ChatCompletion API's (Azure OpenAi and Open AI )

• Add BinaryData to ImageMessage and update callers that use Uri to consider BinaryData when necessary.
• Introduce a simple example of the usage for ImageMessage using the BinaryData constructor overload.
• Update unit tests to reflect the change.

Related issue number

Closes #1893

Checks

• [ ] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.

[gitguardian[bot]]

⚠️ GitGuardian has uncovered 40 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

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