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/Test-ConfigurationDataMissingReferenceError.ps1

@@ -0,0 +1,11 @@
+function Test-ConfigurationDataMissingReferenceError {
+    [CmdletBinding()]
+    Param(
+        [Parameter(Mandatory=$true)]
+        [System.Management.Automation.ErrorRecord]
+        $ErrorRecord
+    )
+
+    $Message = $ErrorRecord.Exception.Message
+    return $Message -match "^(Parameter|Variable) \[.+\] is not defined\.$"
+}