Unify how --format is handled by commands

[fabpot]

Q	A
Branch?	7.2
Bug fix?	no
New feature?	yes
Deprecations?	yes
Issues	n/a
License	MIT

We have many commands that have a --format option. This PR tries to unify the way we handle them:

• Deprecate the text format (used only twice) in favor of the more common txt one;
• Add auto-completion for --format in all commands;
• Add help about the --format option in all commands that supports it (in a unified way)

Side note: To avoid confusion, I think we should rename the --format option for the uuid:generate and ulid:generate commands as it does something different from the commonly used --format option.