Add support for bash completion

[jvalkeal]

Once we're starting to get better support for spring-native resulting proper OS level commands it makes a lot of sense to have support to create completion scripts for shells. I've been looking how things are generated from a go side i.e. for bash as some are based on https://github.com/spf13/cobra which have support for bash, zsh, fish and PowerShell.

We could shamelessly copy how actual completion scripts are constructed(as that's the bread and butter) and make that to work with a bit of template magic around https://github.com/antlr/stringtemplate4 which seems to work nicely with native stuff. This all as a result of a quick experiment of making generic completion bash command.