add caddy server as system install

[nitishkumar71]

Description

Motivation and Context

• I have raised an issue to propose this change, which has been given a label of design/approved by a maintainer (required)
  Closes Add Caddy Server as System install #1069

How Has This Been Tested?

Executed the command arkade system install caddy produced below output

Installing Caddy Server to /usr/local/bin
Installing version: v2.7.6 for: amd64
Downloading from: https://github.com/caddyserver/caddy/releases/download/v2.7.6/caddy_2.7.6_linux_amd64.tar.gz
13.98 MiB / 13.98 MiB [--------------------------------------------------------------------------------------------------------------------------------------------------------------] 100.00%
Downloaded to: /tmp/arkade-345065249/caddy_2.7.6_linux_amd64.tar.gz
Unpacking Caddy to: /usr/local/bin/caddy

If updating or adding a new CLI to arkade get, run:

go build && ./hack/test-tool.sh TOOL_NAME

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

• I have updated the list of tools in README.md if (required) with ./arkade get --format markdown
• I have updated the list of apps in README.md if (required) with ./arkade install --help

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s

• I have tested this on arm, or have added code to prevent deployment

[alexellis]

It's not the usual pattern, but Caddy installs to /usr/bin/

Have a look at the faasd script to see what else may be missing, including adding users etc.

For adding users, we will need to detect the OS from /etc/os-release and reject the command if it's not debian or ubuntu.

In the future, if someone needs this for Alpine or Fedora, they can add a patch to run useradd with the correct syntax for their distro.

There are also some directories created and chmod is run.

[alexellis]

Good start, but there's more needed for a system install to match how Caddy is installed with faasd: https://github.com/openfaas/faasd/blob/master/hack/install.sh