Implement Get-AzWebSiteLog for Azure Powershell module

## Description of the new feature
Basically, the Azure Powershell module doesn't have this function promised, in this documents.

https://docs.microsoft.com/en-us/azure/azure-functions/functions-monitoring?tabs=cmd

I filed an issue for the webpage, then was confirmed the function is missing and was asked to request this feature.

https://github.com/MicrosoftDocs/azure-docs/issues/57766
## Proposed implementation details (optional)

Start with the Azure Functions monitoring documentation linked in the issue and compare its promised Get-AzWebSiteLog command with the existing Azure PowerShell WebSite cmdlets. The issue names no repository files, tests, or entry point, so locate the related cmdlet module and its test coverage before defining the implementation. Done means the command is available and its behavior matches the documented log retrieval.

Written by the indexing model from the issue text.