editoast: add --skip-if-exists to user add cli

[flomonster]

This fixes crash loop from editoast using docker compose.

Example:

$ editoast user add --help
Add a user

Usage: editoast user add [OPTIONS] [NAME] [IDENTITIES]...

Arguments:
  [NAME]           Name of the user
  [IDENTITIES]...  Identities of the user

Options:
  -s, --skip-if-exists  Skip if one of the identities already exists
  -h, --help            Print help
$ editoast user add -s 'Lol' 'lol2' 'mock/mocked'
Skipped: Identity mock/mocked already exists

[younesschrifi]

LGTM

[leovalais]

Suggested change

	#[arg(short, long)]
	#[arg(long)]

nit: -s for "skip" is unusual I think