feat: bash completion

[ankush]

Complete following things:

• site names
• command/options

[gavindsouza]

Adding some reference of past efforts - I had a half-baked zsh script that I used for site completions (https://github.com/gavindsouza/bench-autocomplete), the idea then was to built a template script that (re)generates the autocompletion file which was a lot faster than dealing with https://github.com/frappe/bench/blob/develop/completion.sh