Refactor SSH configuration and key management

- Restrict to specific key exchanges / MACs / ciphers.
- Refactored GetSSHKey method to return an ssh.Signer instead of byte array.
- Added common package.

Co-authored-by: nhas <jordanatararimu@gmail.com>

beszel/internal/common/common.go

@@ -0,0 +1,7 @@
+package common
+
+var (
+	DefaultKeyExchanges = []string{"curve25519-sha256"}
+	DefaultMACs         = []string{"hmac-sha2-256-etm@openssh.com"}
+	DefaultCiphers      = []string{"chacha20-poly1305@openssh.com"}
+)