Handle tags when converting policies to rules

**Description of the issue**

Tags are not handled by `Export-AzPolicyAssignmentRuleData`

**To Reproduce**

Steps to reproduce the issue:

Run `Get-AzPolicyAssignmentDataSource | Export-AzPolicyAssignmentRuleData -Verbose` against an assignment file with this content:

```
[
{
"Identity": null,
"Location": null,
"Name": "12e4c75dcb1d3c43",
"ResourceId": "...",
"ResourceName": "12e4c75dcb1d3c43",
"ResourceGroupName": null,
"ResourceType": "Microsoft.Authorization/policyAssignments",
"SubscriptionId": null,
"Sku": null,
"PolicyAssignmentId": "...",
"Properties": {
"Scope": "...",
"NotScopes": null,
"DisplayName": "...",
"Description": "...",
"Metadata": {
"createdBy": "d5f68f99-e07e-4453-add5-cebdba7b3c92",
"createdOn": "2020-05-12T22:14:43.4731105Z",
"updatedBy": "d5f68f99-e07e-4453-add5-cebdba7b3c92",
"updatedOn": "2022-05-10T22:06:19.9892111Z"
},
"EnforcementMode": 0,
"PolicyDefinitionId": "...",
"Parameters": {
"allowedLocations": {
"value": [
"centraluseuap",
"eastus2euap",
"westcentralus",
"northcentralus",
"westus",
"eastasia",
"australiacentral",
"australiaeast",
"canadacentral",
"northeurope",
"japaneast",
"koreacentral",
"uksouth",
"centralus",
"eastus",
"westus2",
"francecentral",
"southafricanorth",
"germanynorth",
"norwayeast",
"switzerlandnorth",
"uaenorth",
"southeastasia",
"australiacentral2",
"australiasoutheast",
"canadaeast",
"westeurope",
"japanwest",
"koreasouth",
"ukwest",
"eastus2",
"southcentralus",
"brazilsouth",
"francesouth",
"southafricawest",
"germanywestcentral",
"norwaywest",
"uaecentral",
"switzerlandwest",
"brazilus",
"global",
"brazilsoutheast",
"westus3",
"swedencentral",
"swedensouth",
"jioindiawest",
"jioindiacentral",
"centralindia",
"southindia",
"westindia",
"qatarcentral",
"qatarcentral2",
"eastusslv",
"israelcentral",
"polandcentral",
"taiwannorth",
"taiwannorthwest",
"italynorth"
]
},
"effect": {
"value": "AuditIfNotExists"
},
"tagname": {
"value": "SQLADSExcluded"
}
},
"NonComplianceMessages": null
},
"PolicyDefinitions": [
{
"Name": "386c8ebacef0fc43",
"ResourceId": "...",
"ResourceName": "386c8ebacef0fc43",
"ResourceType": "Microsoft.Authorization/policyDefinitions",
"SubscriptionId": null,
"Properties": {
"Description": null,
"DisplayName": "...",
"Metadata": {
"createdBy": "d5f68f99-e07e-4453-add5-cebdba7b3c92",
"createdOn": "2020-05-12T22:14:42.9642707Z",
"updatedBy": null,
"updatedOn": null
},
"Mode": "All",
"Parameters": {
"allowedLocations": {
"type": "Array",
"metadata": {
"displayName": "Allowed locations",
"description": "The list of locations that can be specified when deploying resources.",
"strongType": "location"
}
},
"tagname": {
"type": "String",
"metadata": {
"displayName": "Exclusion Tag Name",
"description": "Rule is not deployed if this tag exists on the SQL Server"
}
},
"effect": {
"type": "String",
"metadata": {
"displayName": "Effect",
"description": "Enable or disable the execution of the policy"
},
"allowedValues": [
"AuditIfNotExists",
"Disabled"
],
"defaultValue": "AuditIfNotExists"
}
},
"PolicyRule": {
"if": {
"allOf": [
{
"field": "type",
"equals": "Microsoft.Sql/servers"
},
{
"field": "location",
"in": "[parameters(\u0027allowedLocations\u0027)]"
},
{
"field": "[concat(\u0027tags[\u0027, parameters(\u0027tagname\u0027), \u0027]\u0027)]",
"exists": "false"
},
{
"value": "[resourceGroup().tags[parameters(\u0027tagname\u0027)]]",
"equals": ""
},
{
"value": "[subscription().tags[parameters(\u0027tagname\u0027)]]",
"equals": ""
}
]
},
"then": {
"effect": "[parameters(\u0027effect\u0027)]",
"details": {
"type": "Microsoft.Sql/servers/securityAlertPolicies",
"name": "default",
"existenceCondition": {
"field": "Microsoft.Sql/servers/securityAlertPolicies/disabledAlerts[*]",
"equals": ""
}
}
}
},
"PolicyType": 1
},
"PolicyDefinitionId": "..."
}
]
}
]
```

I cleaned up all the information that was potentially private replacing it with "..."

**Error output**

```
Export-AzPolicyAssignmentRuleData : An error occurred evaluating expression '[resourceGroup().tags[parameters('tagname')]]' line 161. Object reference not set to an instance of an object.
At C:\Users\verabo.REDMOND\OneDrive - Microsoft\Documents\Policy\WithPSRule\policy.ps1:7 char:36
+ ... ataSource | Export-AzPolicyAssignmentRuleData -OutputPath "rulesFromP ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: ({
"name": "3...onId": "..."
}:JObject) [Export-AzPolicyAssignmentRuleData], ExpressionEvaluationException
+ FullyQualifiedErrorId : System.NullReferenceException,Export-AzPolicyAssignmentRuleData
```

**Module in use and version:**

- Module: PSRule.Rules.Azure
- Version: I ran the version in the main branch up to the latest commit now (https://github.com/Azure/PSRule.Rules.Azure/commit/ef1570a25c6b1850d0f2a9116fa23aaaf13b9157)

Captured output from `$PSVersionTable`:

```text
Name Value
---- -----
PSVersion 5.1.22621.608
PSEdition Desktop
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
BuildVersion 10.0.22621.608
CLRVersion 4.0.30319.42000
WSManStackVersion 3.0
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
```