[Az.RecoveryServices] How can group actions be set programatically on a RSV recovery plan?

### Description

`Get-AzRecoveryServicesAsrRecoveryPlan` already returns information about pre-group and post-group actions that have already been configured. This can be found in the `$Result.Groups[2].StartGroupActions` & `$Result.Groups[2].EndGroupActions` objects.

How can these actions be defined programatically? I have tried changing the values of the in-memory object, hoping to then use `Update-AzRecoveryServicesAsrRecoveryPlan` to submit those changes, but not all of the relevant fields/values can be edited.

In large enterprises, it is unreasonable to use the portal to define these actions; I currently have two actions to define on 68 different vaults (with more coming soon) - surely there must be some way to programatically define the actions, right?

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