[FEATURE REQ] Support empty proxy credentials

[shenglol]

Library name

Azure.Core 1.44.1

Please describe the feature.

We use ContainerRegistryContentClient behind a proxy and encountered a 407 Proxy Authentication Required error. The machine is configured with the HTTP_PROXY, HTTPS_PROXY, or ALL_PROXY environment variables, where the credentials are set to an empty username and password (:). Although the System.Net.Http has supported this behavior since this PR - treating : as CredentialCache.DefaultNetworkCredentials - the Azure SDK for .NET handles proxy credentials with its own parsing logic, which doesn’t follow the same behavior.

Would it be possible to add support for parsing an empty username and password as CredentialCache.DefaultNetworkCredentials to align with the System.Net.Http behavior? This change would prevent the need to configure proxy credentials programmatically in our application.

[github-actions]

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