Add implementation for getStickerSet

[CuriouslyCurious]

It uses a similar structure to how getFile is implemented. Might not be the prettiest implementation but it seemingly works.

Basically you call get_sticker_set() from a sticker object to send a request for the sticker set it is from.

A code snippet that should showcase it working:

use std::env;

use futures::StreamExt;
use telegram_bot::*;

#[tokio::main]
async fn main() -> Result<(), Error> {
    let token = env::var("TELEGRAM_BOT_TOKEN").expect("TELEGRAM_BOT_TOKEN not set");
    let api = Api::new(token);

    // Fetch new updates via long poll method
    let mut stream = api.stream();
    while let Some(update) = stream.next().await {
        // If the received update contains a new message...
        let update = update?;
        if let UpdateKind::Message(message) = update.kind {
            if let MessageKind::Sticker{ ref data, .. } = message.kind {
                // Print received text message to stdout.
                match &data.set_name {
                    Some(_) => {
                        let set = api.send(data.get_sticker_set()).await?;
                        println!("{:?}", set);
                        ()
                    }
                    None => {
                        api.send(message.text_reply(format!(
                            "The sticker you sent me is not part of a sticker set."
                        ))).await?;
                        ()
                    }
                };
            }
        }
    }
    Ok(())
}