RFC: Removal of the ShardScheme::Auto variant

[vilgotf]

Currently ShardScheme (twilight_gateway::cluster::scheme::ShardScheme) has three variants:

1. Auto (auto-sharding)
2. Bucket (very large bots)
3. Range (use range of total shards)

ShardScheme can be set in ClusterBuilder and is then used to construct the shards. So far so good.

ShardScheme is, however, also publicly accessible through the cluster's Config (cluster.config().shard_scheme()), but accessing its information (iter(), from(), total() & to()) requires handling Options when it's the Auto variant. The Auto variant is actually internally translated into the Range variant from Discord's auto-sharding, but this information is then dropped. Exposing Range, instead of Auto on auto-sharding, tells the user what Discord suggested.

Removing the Auto variant is done by storing an Option<ShardScheme> in the ClusterBuilder, and treating None as auto-sharding.

An alternative is to add a auto-sharding: bool field (with an accompanying fn is_auto_sharding(&self) -> bool; method) to ShardScheme::Range to highlight when Range was user specified vs generated by Discord's auto-sharding.

Implementation pr: #1600

[spring4175]

Using an Option<ShardScheme> sounds like the best way to go to me. I may have some opinions about the implementation, but on the surface I think this sounds reasonable.