refactor(agent): Move SSH server into agentssh package (#7004)

Refs: #6177

agent/agentssh/bicopy.go

@@ -0,0 +1,47 @@
+package agentssh
+
+import (
+	"context"
+	"io"
+	"sync"
+)
+
+// Bicopy copies all of the data between the two connections and will close them
+// after one or both of them are done writing. If the context is canceled, both
+// of the connections will be closed.
+func Bicopy(ctx context.Context, c1, c2 io.ReadWriteCloser) {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	defer func() {
+		_ = c1.Close()
+		_ = c2.Close()
+	}()
+
+	var wg sync.WaitGroup
+	copyFunc := func(dst io.WriteCloser, src io.Reader) {
+		defer func() {
+			wg.Done()
+			// If one side of the copy fails, ensure the other one exits as
+			// well.
+			cancel()
+		}()
+		_, _ = io.Copy(dst, src)
+	}
+
+	wg.Add(2)
+	go copyFunc(c1, c2)
+	go copyFunc(c2, c1)
+
+	// Convert waitgroup to a channel so we can also wait on the context.
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		wg.Wait()
+	}()
+
+	select {
+	case <-ctx.Done():
+	case <-done:
+	}
+}