feat(ssh): auto generate additional ssh keys (#33974)

adds capabilities for gitea to generate ecdsa and ed25519 keys by
default
adds cli for built-in ssh key generation helpers


closes: https://github.com/go-gitea/gitea/issues/33783

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/consts/asymkey.go

@@ -0,0 +1,12 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package consts
+
+const (
+	AsymKeyMinBitsRsa = 3071 // 3072-1 to tolerate the leading zero
+	AsymKeyMinBitsEC  = 256
+
+	AsymKeyDefaultBitsRsa   = 4096 // ssh-keygen command defaults to 3072
+	AsymKeyDefaultBitsEcdsa = 256
+)