[Feature]: Add -Identities flag to WebApp Cmdlets

### Description of the new feature

Please add an `-Identities ` flag or similar to `New-AzWebApp`, `New-AzWebAppSlot`, `Set-AzWebApp`, and `Set-AzWebAppSlot` to support managing user-assigned managed identities on web apps and their slots. Consider deprecating/removing `-CopyIdentity` from `New-AzWebAppSlot` which doesn't work.

The ability to manage user assigned managed identities has existed for a while now in `az` CLI ([see here](https://learn.microsoft.com/en-us/cli/azure/webapp/identity?view=azure-cli-latest)) and it makes sense that Powershell should support it as well.

Thanks!

### Proposed implementation details (optional)

Presumably, implementation will be similar to what `az` CLI is using to assign them.

Start with the New-AzWebApp, New-AzWebAppSlot, Set-AzWebApp, and Set-AzWebAppSlot cmdlets, then compare their identity handling with the Azure CLI webapp identity reference linked in the issue. Done means user-assigned managed identities can be managed through an -Identities-like parameter for all four cmdlets, and the non-working -CopyIdentity behavior is addressed.

Written by the indexing model from the issue text.