Create gateway utilization report function

[arudell]

This pull request introduces a new diagnostic feature for monitoring SDN gateway utilization and adds supporting utilities to the codebase. The main changes include a new Show-SdnGatewayUtilization cmdlet for reporting gateway statistics, integration of this cmdlet into the module manifest, and a helper function for formatting kilobit sizes.

New Gateway Utilization Reporting:

• Added the Show-SdnGatewayUtilization cmdlet to provide a detailed, interactive summary of SDN gateway utilization, including statistics on capacity, health, tunnel types, and network connections. The cmdlet supports output to the console with color-coded status and optional export to a JSON report. (src/SdnDiagnostics.psm1, src/SdnDiagnostics.psm1R1208-R1468)
• Registered Show-SdnGatewayUtilization in the module manifest to make it available as part of the module's exported commands. (src/SdnDiagnostics.psd1, src/SdnDiagnostics.psd1R160)

Utility Enhancements:

• Introduced the Format-KiloBitSize helper function to convert kilobit values to MB and GB, used by the new gateway utilization reporting logic. (src/modules/SdnDiag.Utilities.psm1, src/modules/SdnDiag.Utilities.psm1R1096-R1111)

Change type

• Bug fix (non-breaking change)
• Code style update (formatting, local variables)
• New Feature (non-breaking change that adds new functionality without impacting existing)
• Breaking change (fix or feature that may cause functionality impact)
• Other

Checklist:

• My code follows the style and contribution guidelines of this project.
• I have tested and validated my code changes.