credentials: auto-enable VPC endpoint when running in VPC network

[huww98]

What type of PR is this?

/kind feature

What this PR does / why we need it:

When the CSI driver is running in a VPC network, the Alibaba Cloud credentials SDK supports using VPC endpoints for authentication (controlled by the ALIBABA_CLOUD_VPC_ENDPOINT_ENABLED environment variable). Previously, users had to manually set this environment variable to enable VPC endpoint usage.

This PR automatically sets ALIBABA_CLOUD_VPC_ENDPOINT_ENABLED=true when:

1. The current network type is detected as vpc (via utils.GetNetworkType())
2. The environment variable has not already been set by the user

This improves out-of-the-box behavior for VPC deployments without breaking existing configurations where the variable is explicitly set.

Which issue(s) this PR fixes:

No linked issues

Special notes for your reviewer:

The check uses os.LookupEnv to preserve any existing user-set value (including false) and only auto-sets when the variable is absent.

Does this PR introduce a user-facing change?

credentials: automatically enable VPC endpoint for Alibaba Cloud credential provider when running in VPC network

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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This pull-request has been approved by: huww98
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