[Feature Request] Integrate Network Connectivity Center (NCC)

## Feature Description
Replace the use of shared VPC and VPC peering constructs with Network Connectivity Center (NCC) for centralized connectivity management.

## Use Case
The current networking implementation does not leverage NCC, resulting in networking code that is split up by compliance regime rather than being a normalized, centralized capability.

## Proposed Solution
Reformat the network portion of `fast/stages` to use NCC. Normalize the networking code so it is a single stage that takes the regime type (e.g., FedRAMP Moderate, IL5) as an input configuration variable.

## Compliance & Deployment Context
* **Target Deployment Type(s):**
* [ ] US Region Restricted (e.g., Access Policy constraint)
* [x] FedRAMP Medium
* [x] FedRAMP High
* [ ] DoD IL4
* [x] DoD IL5
* [ ] All / General
* **Relevant NIST 800-53r5 Controls:** SC-7 (Boundary Protection).

## Reusability Check
Stellar Engine prioritizes reusability.
* [x] I have checked if this functionality can be achieved by extending an existing module or blueprint.
* [x] I have verified that this does not duplicate existing functionality.

## Alternatives Considered
Maintaining the current state where networking logic is duplicated and hardcoded across different regime-specific folders.

## Additional Context
N/A