Add User Apps

[fgardt]

this is the implementation for user apps that I've been using like this since 3 days after user apps got announced / released to beta.

I like the flexibility that having install_context and interaction_context being separate properties brings but there were concerns regarding usability / it being too complex.

example:

#[poise::command(
    slash_command,
    install_context = "Guild|User",
    interaction_context = "Guild|BotDm|PrivateChannel"
)]
pub async fn info(ctx: Ctx<'_>) -> Result<(), CommandError> {
  // ...command impl...
}

note: this added the unstable_discord_api feature to the serenity dependency, that should probably be removed again(?) I have not found a nice way to feature guard this without having to chain the feature flag in everywhere and some other issues.