Add CLI autocompletion using clap_complete

[titaniumtraveler]

I recently started using sqlx (and sqlx-cli) for a project and
noticed there is no autocompletion.

As it's trivial to add autocompletion to clap-based CLIs and I've done
it a bunch of times I would just create a pr.

How to use it

I added a subcommand named completions to generate completions on the
fly. clap_complete theoretically also supports generating completions
at compile time, but as cargo install doesn't support installing
autocompletion scripts that doesn't really make sense here.

$ sqlx completions bash > path/to/completions

clap_complete per default supports generating autocompletions for bash, elvish, fish, powershell, and zsh.

Sadly there is currently no standard way to create completions for
cargo subcommands, so cargo-sqlx can't benefit from completions.