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/Invoke-ConfigurationDataExpression.ps1

@@ -23,12 +23,40 @@ function Invoke-ConfigurationDataExpression {
         return [bool]::Parse($Expression)
     }
 
+    if(-not (Test-ConfigurationDataExpressionParentheses -Expression $Expression)){
+        throw "Invalid configuration data expression [$Expression]."
+    }
+
     if($Expression -notmatch "^([A-Za-z][A-Za-z0-9]*)\((.*)\)$"){
         throw "Invalid configuration data expression [$Expression]."
     }
 
     $FunctionName = $Matches[1]
     $ArgumentText = $Matches[2]
+
+    if($FunctionName -ieq "coalesce"){
+        $RawArguments = @(Split-ConfigurationDataExpressionArguments -ArgumentText $ArgumentText)
+        Assert-ConfigurationDataExpressionMinimumArgumentCount -Name $FunctionName -Arguments $RawArguments -Count 1
+
+        foreach($Argument in $RawArguments){
+            try {
+                $Value = Invoke-ConfigurationDataExpressionArgument -Argument $Argument -Context $Context
+                if(-not (Test-ConfigurationDataValueIsEmpty -Value $Value)){
+                    return $Value
+                }
+            }
+            catch {
+                if(Test-ConfigurationDataMissingReferenceError -ErrorRecord $_){
+                    continue
+                }
+
+                throw
+            }
+        }
+
+        return $null
+    }
+
     $Arguments = @()
     foreach($Argument in @(Split-ConfigurationDataExpressionArguments -ArgumentText $ArgumentText)){
         $Arguments += ,(Invoke-ConfigurationDataExpressionArgument -Argument $Argument -Context $Context)