Add pure SSH LFS support (#31516)

Fixes #17554
/claim #17554

Docs PR https://gitea.com/gitea/docs/pulls/49

To test, run pushes like: `GIT_TRACE=1` git push. The trace output
should mention "pure SSH connection".

modules/lfstransfer/main.go

@@ -0,0 +1,42 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package lfstransfer
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"code.gitea.io/gitea/modules/lfstransfer/backend"
+
+	"github.com/charmbracelet/git-lfs-transfer/transfer"
+)
+
+func Main(ctx context.Context, repo, verb, token string) error {
+	logger := newLogger()
+	pktline := transfer.NewPktline(os.Stdin, os.Stdout, logger)
+	giteaBackend, err := backend.New(ctx, repo, verb, token, logger)
+	if err != nil {
+		return err
+	}
+
+	for _, cap := range backend.Capabilities {
+		if err := pktline.WritePacketText(cap); err != nil {
+			logger.Log("error sending capability due to error:", err)
+		}
+	}
+	if err := pktline.WriteFlush(); err != nil {
+		logger.Log("error flushing capabilities:", err)
+	}
+	p := transfer.NewProcessor(pktline, giteaBackend, logger)
+	defer logger.Log("done processing commands")
+	switch verb {
+	case "upload":
+		return p.ProcessCommands(transfer.UploadOperation)
+	case "download":
+		return p.ProcessCommands(transfer.DownloadOperation)
+	default:
+		return fmt.Errorf("unknown operation %q", verb)
+	}
+}