Pulling out a bunch of openssh stuff to dedicated package. (#775)

Sources/Packages/Sources/SSHProtocolKit/LengthAndData.swift

@@ -0,0 +1,23 @@
+import Foundation
+
+extension Data {
+
+    /// Creates an OpenSSH protocol style data object, which has a length header, followed by the data payload.
+    /// - Returns: OpenSSH data.
+    package var lengthAndData: Data {
+        let rawLength = UInt32(count)
+        var endian = rawLength.bigEndian
+        return unsafe Data(bytes: &endian, count: MemoryLayout<UInt32>.size) + self
+    }
+
+}
+
+extension String {
+
+    /// Creates an OpenSSH protocol style data object, which has a length header, followed by the data payload.
+    /// - Returns: OpenSSH data.
+    package var lengthAndData: Data {
+        Data(utf8).lengthAndData
+    }
+
+}