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/setting/lfs.go

@@ -13,6 +13,7 @@ import (
 // LFS represents the configuration for Git LFS
 var LFS = struct {
 	StartServer    bool          `ini:"LFS_START_SERVER"`
+	AllowPureSSH   bool          `ini:"LFS_ALLOW_PURE_SSH"`
 	JWTSecretBytes []byte        `ini:"-"`
 	HTTPAuthExpiry time.Duration `ini:"LFS_HTTP_AUTH_EXPIRY"`
 	MaxFileSize    int64         `ini:"LFS_MAX_FILE_SIZE"`