Support flexible coordinate systems for PDF signatures

Added enums `PdfSignatureCoordinateUnit` and `PdfSignatureCoordinateOrigin` to support defining signature coordinates in points or inches and relative to either the bottom-left or top-left corner of the page.

Updated `PdfSignatureAppearanceDto` to include `CoordinateUnit` and `CoordinateOrigin` parameters with default values. Refactored `PdfDigitalSignatureService` to use a new `BuildSignatureBounds` method for calculating signature bounds, which handles unit conversion, origin adjustments, and validation.

Introduced `GetPageHeightInPoints` to retrieve page height for top-left origin calculations. Improved signature field naming precision and added error handling for invalid dimensions and page numbers.

Included a TODO for optimizing the signature check algorithm.

EnvelopeGenerator.Application/Common/Dto/PdfSignatureAppearanceDto.cs

@@ -3,7 +3,38 @@ namespace EnvelopeGenerator.Application.Common.Dto;
 /// <summary>
 /// 
 /// </summary>
-/// <param name="ImageBytes"></param>
+public enum PdfSignatureCoordinateUnit
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    Point = 0,
+
+    /// <summary>
+    /// 
+    /// </summary>
+    Inch = 1
+}
+
+/// <summary>
+/// 
+/// </summary>
+public enum PdfSignatureCoordinateOrigin
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    BottomLeft = 0,
+
+    /// <summary>
+    /// 
+    /// </summary>
+    TopLeft = 1
+}
+
+/// <summary>
+/// 
+/// </summary>
 /// <param name="PageNumber"></param>
 /// <param name="Left"></param>
 /// <param name="Bottom"></param>
@@ -16,6 +47,8 @@ namespace EnvelopeGenerator.Application.Common.Dto;
 /// <param name="ShowLocation"></param>
 /// <param name="ShowDistinguishedName"></param>
 /// <param name="DateTimeFormat"></param>
+/// <param name="CoordinateUnit"></param>
+/// <param name="CoordinateOrigin"></param>
 public sealed record PdfSignatureAppearanceDto(
     int PageNumber,
     double Left,
@@ -28,7 +61,9 @@ public sealed record PdfSignatureAppearanceDto(
     bool ShowReason = false,
     bool ShowLocation = false,
     bool ShowDistinguishedName = false,
-    string DateTimeFormat = "dd.MM.yyyy HH:mm:ss zzz")
+    string DateTimeFormat = "dd.MM.yyyy HH:mm:ss zzz",
+    PdfSignatureCoordinateUnit CoordinateUnit = PdfSignatureCoordinateUnit.Point,
+    PdfSignatureCoordinateOrigin CoordinateOrigin = PdfSignatureCoordinateOrigin.BottomLeft)
 {
     /// <summary>
     ///