Enhance configuration data validation and add new utility functions

- Added validation for empty values in Assert-ConfigurationDataParameter.
- Introduced Assert-ConfigurationDataSecretReference for secret reference validation.
- Implemented Test-ConfigurationDataExpressionParentheses to validate expression syntax.
- Added Test-ConfigurationDataMissingReferenceError to check for missing parameter or variable definitions.
- Updated Invoke-ConfigurationDataExpression to handle new validation logic.
- Revised Readme.md to reflect changes in secret reference handling.

Private/Assert-ConfigurationDataSecretReference.ps1

@@ -0,0 +1,45 @@
+function Assert-ConfigurationDataSecretReference {
+    [CmdletBinding()]
+    Param(
+        [Parameter(Mandatory=$true)]
+        [string]
+        $Name,
+
+        [Parameter(Mandatory=$true)]
+        [ValidateSet("credential", "securestring")]
+        [string]
+        $TypeName,
+
+        [Parameter(Mandatory=$true)]
+        $Value
+    )
+
+    if(-not (Test-ConfigurationDataMap -Value $Value)){
+        return
+    }
+
+    if(-not (Test-ConfigurationDataMapContainsKey -Map $Value -Key "Provider")){
+        throw "Parameter [$Name] uses type [$TypeName] and a secret reference, but [Provider] is not defined."
+    }
+
+    if(-not (Test-ConfigurationDataMapContainsKey -Map $Value -Key "Name")){
+        throw "Parameter [$Name] uses type [$TypeName] and a secret reference, but [Name] is not defined."
+    }
+
+    $Provider = [string](Get-ConfigurationDataMapValue -Map $Value -Key "Provider")
+    if([string]::IsNullOrWhiteSpace($Provider)){
+        throw "Parameter [$Name] secret reference [Provider] must not be empty."
+    }
+
+    $SecretName = [string](Get-ConfigurationDataMapValue -Map $Value -Key "Name")
+    if([string]::IsNullOrWhiteSpace($SecretName)){
+        throw "Parameter [$Name] secret reference [Name] must not be empty."
+    }
+
+    switch($Provider.ToLowerInvariant()){
+        "keepass" { return }
+        default {
+            throw "Parameter [$Name] uses unsupported secret provider [$Provider]."
+        }
+    }
+}