refactor(model)!: flatten `CommandOption`

[vilgotf]

• Simplifies and fixes errors in CommandOption {de,}serialization by delegating to the derive implementation.
• Removes duplicated shared fields for CommandOption and CommandOptionChocie variants.
• Documents all public fields.
• Added a note in the command module recommending users to use the CommandBuilder.
• CommandBuilder retains its API, but is internally more complicated.

Overall this PR reduces internal complexity by a lot.

[vilgotf]

I also want to improve Command's documentation to be like CommandOption's, but I think that should be in a separate PR as this PR is already big enough.

[vilgotf]

Should a follow-up PR be created to sync (duplicate) the docs on CommandOption to the builders in util::builder::command? Perhaps it's easier to cross link to the model docs (as the docs would otherwise risk getting out of sync)?

[7596ff]

I think it would be good to just link to the docs in a follow up, yes

[vilgotf]

@Erk- PTAL