NAT Gateway should be created in the Resource Group of VNet

[nawazkh]

/kind feature

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

• User should be able to create a NAT Gateway in the RG of VNet.
• As of now, the NAT Gateway gets created in the RG of AzureCluster (default behavior).

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

• This appears to be a customer ask. Reference conversation: https://kubernetes.slack.com/archives/CEX9HENG7/p1701694131809119

Environment:

• cluster-api-provider-azure version: v1.12.0
• Kubernetes version: (use kubectl version): n/a
• OS (e.g. from /etc/os-release): n/a