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-ConfigurationDataParameterType.ps1

@@ -48,11 +48,15 @@ function Assert-ConfigurationDataParameterType {
             if(($Value -isnot [string]) -and ($Value -isnot [System.Security.SecureString]) -and (-not (Test-ConfigurationDataMap -Value $Value))){
                 throw "Parameter [$Name] expects type [secureString], but received [$($Value.GetType().Name)]."
             }
+
+            Assert-ConfigurationDataSecretReference -Name $Name -TypeName "securestring" -Value $Value
         }
         "credential" {
             if(($Value -isnot [System.Management.Automation.PSCredential]) -and (-not (Test-ConfigurationDataMap -Value $Value))){
                 throw "Parameter [$Name] expects type [credential], but received [$($Value.GetType().Name)]."
             }
+
+            Assert-ConfigurationDataSecretReference -Name $Name -TypeName "credential" -Value $Value
         }
         default {
             throw "Parameter [$Name] uses unsupported type [$TypeName]."