Add ethtool to docker-platform-monitor (sonic-net#8017)

#### Why I did it
ethtool can be used to query and change settings such as speed, auto- negotiation and checksum offload on many network devices, especially Ethernet devices. 

#### How I did it
add package extension to docker-platform-monitor/Dockerfile.j2

Author: VenkatCisco

dockers/docker-platform-monitor/Dockerfile.j2

@@ -22,6 +22,7 @@ RUN apt-get update &&   \
         rrdtool         \
         python-smbus    \
         python3-smbus   \
+        ethtool         \
         dmidecode       \
         i2c-tools       \
         psmisc          \