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.

Readme.md

@@ -103,7 +103,9 @@ FarmPassphrase = @{
     Required = $true
     Sensitive = $true
     Value = @{
-        SecretName = 'SharePointFarmPassphrase'
+        Provider = 'KeePass'
+        Vault = 'BGW'
+        Name = 'SharePoint/FarmPassphrase'
     }
 }
 
@@ -112,11 +114,21 @@ SetupCredential = @{
     Required = $true
     Sensitive = $true
     Value = @{
-        CredentialName = 'SharePointSetup'
+        Provider = 'KeePass'
+        Vault = 'BGW'
+        Name = 'SharePoint/SetupAccount'
+        UserName = 'BGW\SVC_SHP_SETUP'
     }
 }
 ```
 
+Secret references are validated by `Resolve-DSCConfigurationData`, but they are resolved by `Resolve-DSCConfigurationSecrets` before the normal data resolve step:
+
+```powershell
+$withSecrets = Resolve-DSCConfigurationSecrets -ConfigurationData $merged
+$resolved = Resolve-DSCConfigurationData -ConfigurationData $withSecrets
+```
+
 Array values can be restricted item by item:
 
 ```powershell