Add support for PDF/UA form field

lib/Domain/Builders/MergeOfficeBuilder.cs

@@ -87,6 +87,17 @@ public MergeOfficeBuilder UseNativePdfFormat(PdfFormats format)
         this.Request.UseNativePdfFormat = true;
         this.Request.Format = format;
 
+        return this;
+    }
+
+    /// <summary>
+    ///     This tells gotenberg to enable Universal Access for the resulting PDF.
+    /// </summary>
+    [PublicAPI]
+    public MergeOfficeBuilder EnablePdfUa()
+    {
+        this.Request.EnablePdfUa = true;
+
         return this;
     }
 }

lib/Domain/Requests/MergeOfficeRequest.cs

@@ -59,7 +59,12 @@ protected override string ApiPath
     ///     Note: the documentation says you can't use both together but that regards request headers.
     ///     When true and Format is not set, the client falls back to PDF/A-1a.
     /// </summary>
-    public bool UseNativePdfFormat { get; set; }
+    public bool UseNativePdfFormat { get; set; }
+
+    /// <summary>
+    ///    This tells gotenberg to enable Universal Access for the resulting PDF.
+    /// </summary>
+    public bool EnablePdfUa { get; set; }
 
     protected override IEnumerable<HttpContent> ToHttpContent()
     {

lib/Extensions/MergeOfficeRequestExtensions.cs

@@ -40,6 +40,11 @@ internal static IEnumerable<HttpContent> PropertiesToHttpContent(this MergeOffic
                 request.ExportFormFields.Value,
                 Constants.Gotenberg.LibreOffice.Routes.Convert.ExportFormFields);
 
+        if (request.EnablePdfUa)
+            yield return BuildRequestBase.CreateFormDataItem(
+                "true",
+                Constants.Gotenberg.LibreOffice.Routes.Convert.PdfUa);
+
         if (!request.UseNativePdfFormat && request.Format == default) yield break;
 
         if (!request.UseNativePdfFormat && request.Format != default)

lib/Infrastructure/Constants.cs

@@ -107,6 +107,8 @@ public static class Convert
                     public const string PdfFormat = CrossCutting.PdfFormat;
 
                     public const string Merge = "merge";
+
+                    public const string PdfUa = "pdfua";
                 }
             }
         }