Adding option to specify client telemetry proxy with credentials

[FabianMeiswinkel]

Description

Fix issue: #29023. Customer would like to ONLY enable proxy for their clientTelemetry requests. But currently, clientTelemetryRequests and gatewayRequests are sharing the same HttpClient which makes it impossible. IN PR #29022 we separated the httpclients and allowed customer to config the proxy through System property.
This PR adds the option to also specify proxy credentials.

Example code:

 System.setProperty(
                        "COSMOS.CLIENT_TELEMETRY_PROXY_OPTIONS_CONFIG",
                       "{\"type\":\"HTTP\", \"host\": \"localhost\", \"port\": 8080, \"username\": \"SomeUser\", \"password\": \"SomeSecret\"}");

If an SDK is being regenerated based on a new swagger spec, a link to the pull request containing these swagger spec changes has been included above.

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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[azure-sdk]

API change check

API changes are not detected in this pull request.

[xinlian12]

LGTM, thanks for the quick fix