Add access-type parameter to az apim backup / restore

**Related command**
`az apim backup` [see this docs](https://learn.microsoft.com/de-de/cli/azure/apim?view=azure-cli-latest#az-apim-backup) (And `az apim restore`)

**Is your feature request related to a problem? Please describe.**
As you can see in [the corresponding API documentation](
https://learn.microsoft.com/en-us/azure/api-management/api-management-howto-disaster-recovery-backup-restore?tabs=rest#back-up-an-api-management-service) it is possible to use an `accessKey`, a `
SystemAssignedManagedIdentity` or a `UserAssignedManagedIdentity`. However, the az CLI does not support these options.

**Describe the solution you'd like**
Equally to the PowerShell module (see [here](
https://learn.microsoft.com/en-us/azure/api-management/api-management-howto-disaster-recovery-backup-restore?tabs=powershell#back-up-an-api-management-service) for reference), it would be nice, if the az CLI would implement the `AccessType`-parameter as well.

**Describe alternatives you've considered**
There is no, if you want to use the az CLI and a
`UserAssignedManagedIdentity`.

**Additional context**
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