chore: fix C# build (#31946)

Currently, the generated C# code doesn't compile with this error:

```
error CS0115: 'VpcV2Base._Proxy.Ipv4IpamProvisionedCidrs': no suitable method found to override
```

The reason is that the abstract base class that implements an interface doesn't have an abstract field for that interface. 

Then when a subclass does implement it, it gets rendered to an `override` in C#, but there is no method to override.


----

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packages/@aws-cdk/aws-ec2-alpha/lib/vpc-v2-base.ts

@@ -230,6 +230,13 @@ export abstract class VpcV2Base extends Resource implements IVpcV2 {
   */
   public abstract readonly ownerAccountId: string;
 
+  /**
+   * IPv4 CIDR provisioned under pool
+   * Required to check for overlapping CIDRs after provisioning
+   * is complete under IPAM pool
+   */
+  public abstract readonly ipv4IpamProvisionedCidrs?: string[];
+
   /**
   * If this is set to true, don't error out on trying to select subnets
   */