Add ability to use multiple async runtime

[AS1100K]

Currently, bevy-discord uses tokio runtime as it's default runtime.

One crazy thing that this crate does it, it creates, tokio runtime if not available runtime.rs. I am currently thinking, if this is a good idea or not as libraries should not do that. Any feedback is welcome here.

I was wondering, why not we should support async runtimes like tokio, bevy tasks, etc. Or an another way to deal with this. The problem we face is discord-sdk depends on tokio runtime. The solution here could be using bindgen crate and creating a safe wrapper around it which would integrate well with this feature.

[AS1100K]

The crate discord-sdk depends on tokio runtime and therefore, rich_presence feature will come with tokio runtime while other features will have option to choose different runtime.

I will be opening the PR soon that address this issue