Make toData function public

Changed the accessibility of the `toData` function in `NanoTDF` class from internal to public to allow external modules to access this function. This update is crucial for modules relying on this method to convert NanoTDF objects to raw data.

OpenTDFKit/NanoTDF.swift

@@ -8,7 +8,7 @@ public struct NanoTDF {
     #if DEBUG
         var storedKey: SymmetricKey?
     #endif
-    func toData() -> Data {
+    public func toData() -> Data {
         var data = Data()
         data.append(header.toData())
         data.append(payload.toData())