Support for new activity features

[char-ptr]

Is your feature request related to a problem? Please describe. Not really

Describe the solution you'd like Although the features aren't really new anymore, it would be great to still have the functionality within this library. The two features which i primarily have in mind are the following:

• Asset keys being urls
• Buttons

Describe alternatives you've considered For url asset keys i can bypass the check by setting it not using the method eg:

let mut assets = Assets::default();
assets.large_image = Some(game_thumbnail.clone());
assets.large_text = Some(game_name.clone());

But to get buttons to work i would figure i need to do much more and alter the current source code.

Additional context n/a

[winrg]

this should be done now that #30 is merged

[Jake-Shadle]

Oops