Provide support for additional subnets in an Availability Zone

[rnninad]

How to categorize this issue?
/area control-plane
/area networking
/area performance
/kind enhancement
/platform azure

What would you like to be added:
Currently, in the shoot cluster configuration, the number of zones that can be configured under infrastructureConfig.networks.zones is equal to the number of Availability Zones in a region and each zone (which translates to a subnet) can only have 1 NAT Gateway associated with it. While this set-up works well for most use-cases, business applications that are network-intensive, soon run into the NAT gateway bandwidth limits or connection limits.

Once the NAT Gateway limits are breached, there is no option but to work on a cluster scale-out strategy which is a fairly complicated exercise, which further increases the complexity of the solution.

Hence, if you could provide support for the creation of additional subnets/zones independent of the Availability Zones present in a region, it will allow users to create more NAT Gateways within a cluster, which will ensure the NAT Gateways are not overloaded easily. This approach also aligns with the Azure recommendation - You can split your deployments into multiple subnets and assign each subnet or group of subnets to a NAT gateway to scale out. (link)

Why is this needed:
To ensure users don't run into NAT Gateway limits when running network-intensive applications.