Allow built-in reason to be replaced easily from assertion helper

[BernieWhite]

Currently to add a custom reason using assertion helper methods an implementation might look like this.

# Synopsis: AKS agent pools should run the same Kubernetes version as the cluster
Rule 'Azure.AKS.PoolVersion' {
    $clusterVersion = $TargetObject.Properties.kubernetesVersion
    foreach ($pool in $TargetObject.Properties.agentPoolProfiles) {
        $result = $Assert.HasDefaultValue($pool, 'orchestratorVersion', $clusterVersion);
        $result.AddReason(($LocalizedData.AKSNodePoolVersion -f $pool.name, $pool.orchestratorVersion));
        $result;
    }
}

This can be simplified with a fluent method like:

# Synopsis: AKS agent pools should run the same Kubernetes version as the cluster
Rule 'Azure.AKS.PoolVersion' {
    $clusterVersion = $TargetObject.Properties.kubernetesVersion
    foreach ($pool in $TargetObject.Properties.agentPoolProfiles) {
        $Assert.
            HasDefaultValue($pool, 'orchestratorVersion', $clusterVersion).
            WithReason(($LocalizedData.AKSNodePoolVersion -f $pool.name, $pool.orchestratorVersion));
    }
}

Additionally replacement of reasons is more involved.