feat(gateway,model): Add handling for gateway resume url

[MinnDevelopment]

This adds handling for the new gateway_resume_url, which is provided in the READY dispatch. Resuming should happen on that URL instead of the normal wss://gateway.discord.gg.

I tested this using ClusterBuilder:

let shards = ShardScheme::try_from((0..1, 1))?;
let (cluster, _) = ClusterBuilder::new(token.clone(), Intents::None)
    .shard_scheme(shards)
    .build()
    .await?;

cluster.up().await;

tokio::time::sleep(Duration::from_secs(1)).await;

let hashmap = cluster.down_resumable();

let (cluster, _) = ClusterBuilder::new(token, Intents::None)
    .shard_scheme(shards)
    .resume_sessions(hashmap)
    .build()
    .await?;

cluster.up().await;

Reference: discord/discord-api-docs#5282

[7596ff]

We've added support for this on the next branch, via #1809. However, I think adding this to main is still worth it.

[Erk-]

Just a miniscule nit, otherwise look good to merge