feat: Add show option to the certs plugin

The 'server.key' and 'server.crt' can printed to stdin.
The two options are:
    `dokku certs:show <app> key > server.key`
    `dokku certs:show <app> crt > server.crt`
This will create two files, `server.key` and `server.crt`.
Those files contain the private key and the certificate.

Includes tests.

plugins/certs/help-functions

@@ -34,6 +34,7 @@ fn-help-content() {
     certs:remove <app>, Remove an SSL Endpoint from an app
     certs:report [<app>] [<flag>], Displays an ssl report for one or more apps
     certs:rollback <app>, [NOT IMPLEMENTED] Rollback an SSL Endpoint for an app
+    certs:show <app> <crt|key>, Show the server.crt or server.key on stdout
     certs:update <app> CRT KEY, Update an SSL Endpoint on an app. Can also import from a tarball on stdin
 help_content
 }

plugins/certs/subcommands/show

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
+source "$PLUGIN_AVAILABLE_PATH/certs/functions"
+
+cmd-certs-show() {
+  declare desc="Show the server.crt or server.key on stdout"
+  declare cmd="certs:show"
+  [[ "$1" == "$cmd" ]] && shift 1
+  declare APP="$1" KEY_TYPE="$2"
+
+  [[ -z "$APP" ]] && dokku_log_fail "Please specify an app to run the command on"
+  verify_app_name "$APP"
+  local APP_SSL_PATH="$DOKKU_ROOT/$APP/tls"
+
+  if [[ "$KEY_TYPE" != "key" ]] && [[ "$KEY_TYPE" != "crt" ]]; then
+    dokku_log_fail "specify either 'key' or 'crt'"
+  fi
+
+  if ! is_ssl_enabled "$APP"; then
+    dokku_log_fail "$APP doesn't have an SSL endpoint defined"
+  fi
+
+  cat "$APP_SSL_PATH/server.$KEY_TYPE"
+}
+
+cmd-certs-show "$@"