VPC and Subnet to allow developer to set the Cidr_Block

## :rocket: Feature Request

### General Information

* [ ] :wave: I may be able to implement this feature request

* [x] :warning: This feature might incur a breaking change

### Description

Currently when you create a VPC , you say how many AZ's and the Cidr_mask for each subnet in the respective ZONE , e.g Public, Private....
The Cidr ranges are then automatically assigned, and also not surfaced.

What I need to do is have control over the ISubnets CidrBlock.
Currently you can only say what the Subnet mask is range (/16 - /28), the CDK/ VPC inside CreateSubnets() calculate and assigns the CidrBlock. it does not even report it back #3951

This is great for a simple environment , but in a enterprise environment this just does not work with complex networks, the design will be passed down IP ranges agreed and assigned a head of a single line of code being written.

### Proposed Solution

Allow the enduser/developer/devops person to have the option of assigning the IP ranges (CIRDBlocks) themselves

### Environment

- **CDK CLI Version:** 1.6
- **Module Version:**
- **OS:** all
- **Language:** all

### Other information

Start by reading the VPC CreateSubnets() path and the ISubnet/CidrBlock handling described in the issue. Trace how subnet masks currently produce ranges, then define and validate an option for callers to assign each subnet's CIDR block while preserving automatic allocation. Done means enterprise callers can supply CIDR blocks and existing simple configurations remain supported.

Written by the indexing model from the issue text.