Add supernet address prefix parameter input to Portal UI

[glennmusa]

Description

A recent change #622 introduces a supernet to allow cross-spoke traffic.

This change updates the Portal UI to include an input for this value. It is set to the default in the template.

To demo this: https://portal.azure.com/#blade/Microsoft_Azure_CreateUIDef/CustomDeploymentBlade/uri/https%3A%2F%2Fraw.githubusercontent.com%2FAzure%2Fmissionlz%2Fmain%2Fsrc%2Fbicep%2Fmlz.json/uiFormDefinitionUri/https%3A%2F%2Fraw.githubusercontent.com%2FAzure%2Fmissionlz%2Fglenn%2FaddSupernetToUi%2Fsrc%2Fbicep%2Fform%2Fmlz.portal.json

Issue reference

The issue this PR will close: #608

Checklist

Please make sure you've completed the relevant tasks for this PR out of the following list:

• All acceptance criteria in the backlog item are met
• The documentation is updated to cover any new or changed features
• Manual tests have passed
• Relevant issues are linked to this PR

[Breanna-Stryker]

We did an out of band deep dive on the validation logic, and tested in my environment, works great!