add `attrs` accessor

[vnghia]

For some reasons, one might want to access the inner type language field since the high-level api grammers-client haven't caught up yet. In my case, I want to access the InputStickerSet of the Sticker struct.

I saw that we already have a unstable_raw feature so how about adding something like raw_tl function. I am willing to contribute if that makes sense.

[Lonami]

Reusing the unstable_raw feature to expose raw types is the way to go. Yes please, send a PR. Add what you need under the feature. Although we should probably return the entire raw media, not just attrs.