Expose Shard::handle_event

[ColonelThirtyTwo]

For using Shard manually without a ShardManager: there does not seem to be a public way to tell the shard about heartbeats and other events it needs to continue operating.

ShardRunnder does this via Shard::handle_event but that's pub(crate). Making it pub would at least allow external users to feed events into the Shard.