Add system app for CNI plugins

[alexellis]

Description

Add system app for CNI plugins

Motivation and Context

Required for #654

How Has This Been Tested?

go build && sudo ./arkade system install prometheus
Installing Prometheus to /usr/local/bin
Installing version: v2.34.0 for: amd64
Downloading from: https://github.com/prometheus/prometheus/releases/download/v2.34.0/prometheus-2.34.0.linux-amd64.tar.gz
72.77 MiB / 72.77 MiB [--------------------------------------------------------------------------------------------------------------------------------------------------------------] 100.00%
Downloaded to: /tmp/prometheus-2.34.0.linux-amd64.tar.gz
Unpacking binaries to: /tmp/prometheus2922357963
Copying binaries to: /usr/local/bin

sudo chmod +x /usr/local/bin/prometheus 

/usr/local/bin/prometheus --version
prometheus, version 2.34.0 (branch: HEAD, revision: 881111fec4332c33094a6fb2680c71fffc427275)
  build user:       root@121ad7ea5487
  build date:       20220315-15:18:00
  go version:       go1.17.8
  platform:         linux/amd64

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Documentation

App still needs adding to README, along with others added recently.