[Feature]: Optionally run `Backup-AzApiManagement` without waiting

### Description of the new feature

`Backup-AzApiManagement` can in my experience take 30 minutes or more per APIM instance. This is using managed identity like my example from comment .

It would be great if there were a switch to run this command async, like `-NoWait`, `-AsJob` or something like that. For now I'll `Start-ThreadJob`, wait some time and check job state, before assuming the backup will succeed and then exit the script. Else this Automation Account runbook job will run for hours ($$ and not supported per ), were all it does is waiting for `Backup-AzApiManagement` to finish.

API doc, seems it supports async / skip wait for it to finish? Ref HTTP return code 202 for `Accepted: Location header contains the URL where the status of the long running operation can be checked.`:

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### Proposed implementation details (optional)

```powershell
# Start backup, don't wait for it to finish
$BackupJob = Backup-AzApiManagement @SplatRequiredParameters -NoWait

<#
API returns 202 with URL to check status. Status can then be checked with:
#>

# Get current status of the backup job
Get-AzApiBackupStatus -BackupJob $BackupJob
```

Start with the Backup-AzApiManagement cmdlet and the linked API Management backup REST documentation, focusing on the 202 response and its Location header. Define how the proposed no-wait result can be checked through Get-AzApiBackupStatus, and consider the command's completion behavior as the acceptance criteria.

Written by the indexing model from the issue text.