Add Golang code for generating SSH key

Add Golang code example for generating SSH key along with associated documentation.

Signed-off-by: Scott S. Lowe <scott.lowe@scottlowe.org>

golang/README.md

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+# Learning Tools: Golang
+
+The contents of this folder are focused on improving knowledge in programming with Golang.
+
+## Contents
+
+**generate-ssh-key**: This folder provides some code for generating SSH keys in Golang.

golang/generate-ssh-key/README.md

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+# Generate an SSH Key in Golang
+
+The code in this directory was heavily derived from [this StackOverflow page](https://stackoverflow.com/questions/21151714/go-generate-an-ssh-public-key).
+
+This was written and tested with Golang 1.18 on an ARM64-based Mac, but it should run fine on any platform or architecture supported by Go.

golang/generate-ssh-key/go.mod

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+module go-sshkey
+
+go 1.18
+
+require golang.org/x/crypto v0.7.0
+
+require golang.org/x/sys v0.6.0 // indirect

golang/generate-ssh-key/main.go

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+package main
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"log"
+
+	"golang.org/x/crypto/ssh"
+)
+
+func main() {
+	// Generate a 4096-bit RSA private key
+	privateKey, err := rsa.GenerateKey(rand.Reader, 4096)
+	if err != nil {
+		log.Printf("error encountered: %s", err.Error())
+	}
+	privateKeyDer := x509.MarshalPKCS1PrivateKey(privateKey)
+	privateKeyBlock := pem.Block{
+		Type:    "RSA PRIVATE KEY",
+		Headers: nil,
+		Bytes:   privateKeyDer,
+	}
+	privateKeyPem := string(pem.EncodeToMemory(&privateKeyBlock))
+	fmt.Println(string(privateKeyPem))
+
+	// Generate a matching public key
+	publicKey := privateKey.PublicKey
+	publicKeyDer, err := x509.MarshalPKIXPublicKey(&publicKey)
+	if err != nil {
+		log.Printf("error encountered: %s", err.Error())
+	}
+	publicKeyBlock := pem.Block{
+		Type:    "PUBLIC KEY",
+		Headers: nil,
+		Bytes:   publicKeyDer,
+	}
+	publicKeyPem := string(pem.EncodeToMemory(&publicKeyBlock))
+	fmt.Println(string(publicKeyPem))
+
+	// Generate the public key in OpenSSH authorized_keys format
+	sshPubKey, err := ssh.NewPublicKey(&publicKey)
+	if err != nil {
+		log.Printf("error encountered: %s", err.Error())
+	}
+	fmt.Println(string(ssh.MarshalAuthorizedKey(sshPubKey)))
+}