Add validation for existing vnet and subnet

[CecileRobertMichon]

/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]

When an existing vnet & subnets are provided as part of the custom vnet feature, we should validate that the existing vnet and subnets are valid in a validation webhook (part of the AzureCluster webhook).

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• cluster-api-provider-azure version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[CecileRobertMichon]

/help
/good-first-issue

[k8s-ci-robot]

@CecileRobertMichon:
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In response to this:

/help
/good-first-issue

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[weng271190436]

/assign