Add ssh completion (#891)

Read ssh config from `/etc/ssh/ssh_config` and `~/.ssh/config`.

Given: 
![螢幕擷取畫面 2024-07-09
232722](https://github.com/nushell/nu_scripts/assets/5771562/9294efd7-42c0-4a6c-9dac-06a6a87f67c5)

Will show:

![螢幕擷取畫面 2024-07-09
232640](https://github.com/nushell/nu_scripts/assets/5771562/a8e3844b-ac91-417b-a973-c31366ae1460)

custom-completions/ssh/README.md

@@ -0,0 +1,40 @@
+# SSH completions
+
+A Nushell extern definition and completers for the ssh command.
+
+This module provides extern definitions for most of the ssh command options and flags.
+
+## Usage
+
+Simply import the extern definitions with
+
+```nu
+source path/to/ssh-completions.nu
+```
+
+This script will parse `/etc/ssh/ssh_config` and `~/.ssh/config` to fetch SSH config hosts.
+
+Given the following config:
+
+```
+Host my-ip
+    HostName 192.168.50.237
+Host mydomain
+    HostName mydomain.example.com
+Host my-domain-2
+    HostName mydomain-2.example.com
+Host my_domain_3
+    HostName mydomain_3.example.com
+```
+
+
+When you press the tab key, it will display:
+
+```
+❯ | ssh
+my-ip             192.168.50.237
+mydomain          mydomain.example.com
+my-domain-2       mydomain-2.example.com
+my_domain_3       mydomain_3.example.com
+
+```

custom-completions/ssh/ssh-completions.nu

@@ -0,0 +1,52 @@
+export extern "ssh" [
+    destination?: string@"nu-complete ssh-host"
+    -4            # Forces ssh to use IPv4 addresses only.
+    -6            # Forces ssh to use IPv6 addresses only.
+    -A            # Enables forwarding of connections from an authentication agent such as ssh-agent(1).
+    -a            # Disables forwarding of the authentication agent connection.
+    -B: string    # bind_interface
+    -b: string    # bind_address
+    -C            # Requests compression of all data
+    -c: string    # cipher_spec
+    -D            # [bind_address:]port
+    -E: string    # log_file
+    -e            # escape_char
+    -F: string    # configfile
+    -f            # Requests ssh to go to background just before command execution.
+    -G            # Causes ssh to print its configuration after evaluating Host and Match blocks and exit.
+    -g            # Allows remote hosts to connect to local forwarded ports
+    -I: string    # pkcs11
+    -i: string    # identity_file
+    -J: string    # destination
+    -K            # Enables GSSAPI-based authentication and forwarding(delegation) of GSSAPI credentials to the server.
+    -k            # Disables forwarding (delegation) of GSSAPI credentials to the server.
+    -L: string    # [bind_address:]port:host:hostport / [bind_address:]port:remote_socket / local_socket:host:hostport / local_socket:remote_socket
+    -l: string    # login_name
+    -M            # Places the ssh client into “master” mode for connection sharing.
+    -m: string    # mac_spec
+    -N            # Do not execute a remote command.
+    -n            # Redirects stdin from /dev/null (5) for details.
+    -O: string    # ctl_cmd
+    -o: string    # option
+]
+
+def "nu-complete ssh-host" [] {
+    let files = [
+        '/etc/ssh/ssh_config',
+        '~/.ssh/config'
+    ] | filter { |file| $file | path exists } 
+
+    $files | each { |file|
+        let lines = $file | open | lines | str trim
+        let hosts = $lines
+        | parse --regex '^Host\s+(?<host>.+)'
+        | get host
+
+        let hostnames = $lines
+        | parse --regex '^HostName\s+(?<hostname>.+)'
+        | get hostname
+        $hosts | zip $hostnames | each { || 
+            {'value': $in.0, 'description': $in.1}
+        }
+    } | flatten
+}