Add Unit Property to RunbookRetentionPeriod in Octopus.Client to Align with Server Contract

[enf0rc3]

Background

The current implementation of the RunbookRetentionPeriod model in Octopus.Client does not fully align with the server's data contract. Specifically, it lacks the Unit property, which the server supports.

Result

This PR introduces the Unit property as an enum to the RunbookRetentionPeriod model in Octopus.Client to match the server's contract.

Additional Details

Current RunbookRetentionPeriod in Octopus.Client:

namespace Octopus.Client.Model
{
    public class RunbookRetentionPeriod
    {
        public int QuantityToKeep { get; set; }

        public bool ShouldKeepForever { get; set; }
    }
}

Current RunbookRetentionPeriod on Server (as per Swagger documentation ):

RunbookRetentionPeriod{
  QuantityToKeep	integer($int32)
  ShouldKeepForever	boolean
  readOnly: true
  Unit	string
}

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[desruc]

Looks good. Thanks for aligning these.