use vpc endpoint

[ethan256]

When the service has access to the AliCloud VPC network, it should prioritize getting STS credentials through the VPC network rather than accessing the sts endpoints through the public network.

Therefore, I think we should provide an opportunity to allow users to connect to VPC endpoints instead of brute-force hard-coding a public network endpoint.

[ethan256]

Code: https://github.com/hashicorp/vault-plugin-secrets-alicloud/blob/main/clients/sts.go#L15
alicloud sts endpoint doc: https://www.alibabacloud.com/help/en/resource-access-management/latest/api-doc-sts-2015-04-01-endpoint