Refactor Initialize-NatNetwork missing CNI plugin installation (3/4)

[TinaMor]

PR description

Refactor initialize-NatNetwork command to:

1. Check (and install) missing plugins first before checking if the network exists.
2. Default the WinCNI Path to $ENV:Programfiles\containerd\cni

Testing information

> Initialize-NatNetwork -Force -Confirm:$false -Gateway 192.168.0.1

Creating NAT network
WARNING: Windows CNI plugins have not been installed. Installing Windows CNI plugins at 'C:\Program Files\containerd\cni'
Downloading CNI plugin version 0.3.1 at C:\Program Files\containerd\cni
Extracting WinCNIPlugin to C:\Program Files\containerd\cni\bin
Cleanup to remove downloaded files
CNI plugin version 0.3.1 (microsoft/windows-container-networking) successfully installed at C:\Program Files\containerd\cni
WARNING: nat already exists. To view existing networks, use "Get-HnsNetwork". To remove the existing network use the "Remove-HNSNetwork" command.

Checklist

As part of our commitment to engineering excellence, before submitting this PR, please make sure:

• You've tested this code in both Desktop & Server environments and AMD & ARM64 enviroments (functional testing).
• You've added unit tests for new code.
• You've added/updated documentation in the cmdlet docs, command-reference.md and the modules help files.
• You've reviewed the PR/code best practices defined in the CONTRIBUTING.md.

In addition, after this PR has been reviewed, please agree to:

• If changes have been made to your PR in the process of addressing comments, please make sure to test again the final version in both AMD and ARM64 environments.
• Validate your changes have not introduced any regressions.

[iankingori]

Why did we remove the consent prompt for additional installation when -force is used?

[TinaMor]

We implemented the function using ShouldProcess() which handles -Confirm flag. This then became redundant.

$PSCmdlet.ShouldProcess