Add systemd unit as user contrib (#759)

* Add systemd unit as user contrib

Signed-off-by: Samveen <samveen@samveen.in>

* Fix minor issues exposed by review and linter run

Signed-off-by: Samveen <samveen@samveen.in>

---------

Signed-off-by: Samveen <samveen@samveen.in>

Author: samveen

contrib/systemd/README.md

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+# Introduction
+
+This is a starter systemd unit and associated install instructions, to save new users the time of building up everything
+from scratch.
+
+This was created and works as intended on Ubuntu 22.04 LTS (Jammy Jellyfish)
+
+# Motivations
+
+- Save researching time for new users
+- Adding common starting point for deployment files for distributions, if, as and when kafdrop gets included into
+  packaging systems by linux distributions further downstream.
+
+*Note: I could also contribute packaging information for rpm and deb formats, if that's something that'll be useful.*
+
+# Installation steps
+
+***Note: The assumed path of installation ($INSTALLDIR) is `/opt/kafdrop`. If any other path is used, then the service
+file needs to be modified to use that path in the `# Paths` commented section.***
+
+1. Create the directory `$INSTALLDIR` and download the latest release there.
+
+2. Create the kafdrop user and group:
+```
+systemd-sysusers --inline 'u  kafdrop  -  "KafDrop user"  $INSTALLDIR  /usr/sbin/nologin'
+```
+
+3. Copy `start.sh` to `$INSTALLDIR`, edit it to fix the startup parameters and options for your use case, and give it
+   execute permissions
+```
+chmod 755 $INSTALLDIR/start.sh
+```
+
+4. Copy the `kafdrop.service` file to `$INSTALLDIR` and create a link to it in `/etc/systemd/system`
+```
+(cd /etc/systemd/system && ln -s $INSTALLDIR/kafdrop.service)
+```
+
+5. Refresh systemd runtime configuration.
+```
+systemctl daemon-reload
+```
+
+6. Enable and start service.
+```
+systemctl enable kafdrop.service  && systemctl start kafdrop.service
+```
+
+7. Profit!!

contrib/systemd/kafdrop.service

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+# Systemd unit file for KafDrop
+# Thanks to Kafka users on the internet for the starter template
+
+[Unit]
+Description=KafDrop server
+Documentation=https://github.com/obsidiandynamics/kafdrop
+Requires=network.target remote-fs.target
+After=network.target remote-fs.target kafka.target
+
+[Service]
+Type=simple
+ExecStart=/opt/kafdrop/start.sh
+SuccessExitStatus=143
+Restart=on-abnormal
+# Logging
+SyslogIdentifier=kafdrop
+# Paths
+User=kafdrop
+Group=kafdrop
+PrivateTmp=yes
+# Network service
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+# Restrict privileges
+NoNewPrivileges=true
+# paths
+WorkingDirectory=/opt/kafdrop
+ReadOnlyPaths=/opt/kafdrop
+RuntimeDirectory=kafdrop
+
+[Install]
+WantedBy=multi-user.target

contrib/systemd/start.sh

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+#!/usr/bin/env bash
+
+# Variables for single point of customization
+BOOTSTRAP_SERVER="localhost:9092"
+VERSION="4.1.0"
+LISTEN_PORT=8080
+
+# Running from JAR
+exec java --add-opens=java.base/sun.nio.ch=ALL-UNNAMED \
+    -jar kafdrop-"${VERSION}".jar \
+    --kafka.brokerConnect="${BOOTSTRAP_SERVER}" \
+    --server.port=${LISTEN_PORT} --management.server.port=${LISTEN_PORT}