Add close method to IO writer protocol (#336)

Add close method to IO writer protocol and update testing BufferWriter
to conform to the protocol

Signed-off-by: Kathryn Baldauf <k_baldauf@apple.com>

Sources/Containerization/IO/Writer.swift

@@ -19,4 +19,5 @@ import Foundation
 /// A type that writes the provided Data.
 public protocol Writer: Sendable {
     func write(_ data: Data) throws
+    func close() throws
 }

Sources/Integration/ContainerTests.swift

@@ -72,6 +72,10 @@ extension IntegrationSuite {
             }
             self.data.append(data)
         }
+
+        func close() throws {
+            return
+        }
     }
 
     final class StdinBuffer: ReaderStream {