[Feature] Download kubeseal using Golang

[alexellis]

Expected Behaviour

kubeseal is required for use with faas-cli cloud seal

We could add a command like this:

faas-cli download kubeseal

Current Behaviour

There's manual instructions in the README on how to get kubeseal

Possible Solution

Follow the approach taken by k3sup to determine architecture and download the binary to:

~/.openfaas/bin/

https://github.com/alexellis/k3sup/blob/master/pkg/cmd/openfaas_app.go#L65

We will also need to print a command like:

# Add $HOME/.openfaas/bin/ to your $PATH variable

export PATH=$PATH:$HOME/.openfaas/bin

[utsavanand2]

Derek assign: me