Generate the autocompletion script for bash

Generate the autocompletion script for the bash shell.

This script depends on the 'bash-completion' package. If it is not installed already, you can install it via your OS's package manager.

To load completions in your current shell session:

source <(azcopy completion bash)

To load completions for every new session, execute once:

azcopy completion bash > /etc/bash_completion.d/azcopy