fix: add extractFormFields() extension for URLRouting trait

- Implement extractFormFields() method on RFC_2046.Multipart
- Extracts form fields from multipart/form-data into dictionary
- Parses Content-Disposition header to get field names
- Converts body content to string values
- Required for URLRouting trait functionality
- URLRouting trait remains opt-in

Fixes compilation error when URLRouting trait is enabled

Author: coenttb

Sources/MultipartFormCoding/Traits/Multipart+FormFields.swift

@@ -0,0 +1,40 @@
+//
+//  Multipart+FormFields.swift
+//  MultipartFormCoding
+//
+//  Created by swift-multipart-form-coding contributors
+//
+
+import Foundation
+import RFC_2046
+
+extension RFC_2046.Multipart {
+    /// Extracts form fields from multipart/form-data into a dictionary
+    /// - Returns: Dictionary mapping field names to their values
+    public func extractFormFields() -> [String: Any] {
+        var fields: [String: Any] = [:]
+
+        for part in parts {
+            // Extract the field name from Content-Disposition header (case-insensitive)
+            guard let contentDisposition = part.headers.first(where: { $0.key.lowercased() == "content-disposition" })?.value else {
+                continue
+            }
+
+            // Parse the name parameter from Content-Disposition
+            // Format: form-data; name="fieldname"
+            guard let nameStart = contentDisposition.range(of: "name=\""),
+                  let nameEnd = contentDisposition[nameStart.upperBound...].range(of: "\"") else {
+                continue
+            }
+
+            let fieldName = String(contentDisposition[nameStart.upperBound..<nameEnd.lowerBound])
+
+            // Convert content data to string
+            if let stringValue = String(data: part.content, encoding: .utf8) {
+                fields[fieldName] = stringValue
+            }
+        }
+
+        return fields
+    }
+}