Add acme-keygen tool to generate ACME account keys

Create command-line tool to generate and register ACME account private
keys with Let's Encrypt. Outputs PEM-encoded key for use in config file
to maintain persistent account across deployments.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

cmd/acme-keygen/main.go

@@ -0,0 +1,83 @@
+// Copyright (c) 2025 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"os"
+
+	"github.com/getprobo/probo/pkg/crypto/keys"
+	"github.com/getprobo/probo/pkg/crypto/pem"
+	"golang.org/x/crypto/acme"
+)
+
+func main() {
+	var (
+		email     = flag.String("email", "", "Email address for ACME account (required)")
+		keyType   = flag.String("key-type", "EC256", "Key type: EC256, EC384, RSA2048, RSA4096")
+		directory = flag.String("directory", "https://acme-v02.api.letsencrypt.org/directory", "ACME directory URL")
+	)
+
+	flag.Parse()
+
+	if *email == "" {
+		fmt.Fprintln(os.Stderr, "Error: -email is required")
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	accountKey, err := keys.Generate(keys.Type(*keyType))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error generating key: %v\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Fprintf(os.Stderr, "Generated %s account key\n", *keyType)
+
+	client := &acme.Client{
+		Key:          accountKey,
+		DirectoryURL: *directory,
+	}
+
+	ctx := context.Background()
+	account := &acme.Account{
+		Contact: []string{"mailto:" + *email},
+	}
+
+	registeredAccount, err := client.Register(ctx, account, acme.AcceptTOS)
+	if err != nil {
+		if err == acme.ErrAccountAlreadyExists {
+			fmt.Fprintf(os.Stderr, "Account already exists for this key\n")
+		} else {
+			fmt.Fprintf(os.Stderr, "Error registering account: %v\n", err)
+			os.Exit(1)
+		}
+	} else {
+		fmt.Fprintf(os.Stderr, "Successfully registered ACME account\n")
+		fmt.Fprintf(os.Stderr, "Account URI: %s\n", registeredAccount.URI)
+	}
+
+	keyPEM, err := pem.EncodePrivateKey(accountKey)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error encoding key: %v\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Fprintf(os.Stderr, "\n=== ACME Account Private Key (PEM) ===\n")
+	fmt.Fprintf(os.Stderr, "Add this to your configuration under custom-domains.acme.account-key:\n\n")
+	fmt.Print(string(keyPEM))
+}