Add allowed_mentions to `ClientBuilder`

[Khsmty]

description:

Currently, there is no allowed_mentions in the ClientBuilder, so it is necessary to specify allowed_mentions for each message to be sent. It would be useful if it could be specified like FrameworkOptions in poise.

impression:

let client = Client::builder(token, intents)
    .allowed_mentions(
        CreateAllowedMentions::new()
            .all_users(false)
            .all_roles(false)
            .everyone(false)
            .replied_user(false)
    )
    .await
    .unwrap()