Add helper methods to AssignedCollators to avoid is_empty checks

[tmpolaczyk]

AssignedCollators struct has this field:

container_chains: BTreeMap<ParaId, Vec<AccountId>>,

It is a map of container chain para id to list of collators assigned to that chain. A common query is "does this chain have any assigned collators?". The naive solution is to do container_chains.contains(para_id), but that's wrong because the chain could be in the BTreeMap but with an empty collator list. The correct solution is something like container_chains.get(para_id).unwrap_or(&vec![]).is_empty() == false, which is more complex and easy to forget.

This PR tries to avoid that problem by making the container_chains field private, and only allow reading through getters that automatically check the empty vector case and convert it into None. Example:

/// Get collators assigned to container chain `para_id`. Handles the edge case of an empty list.
/// If this returns Some, the Vec can be assumed to not be empty.
pub fn get_container_chain(&self, para_id: &ParaId) -> Option<&Vec<AccountId>> {
    let x = self.container_chains.get(para_id);

    // Filter out empty assignment, return None instead
    match x {
        Some(x) if x.is_empty() => None,
        x => x,
    }
}

And now callers can simply do assigned_collators.get_container_chain(para_id).is_some() to check if a chain has any collators.

Also added more helper methods that are needed because the field is private.

[tmpolaczyk]

On hold because making the field private made too many tests fail to compile, but I can't think of any better alternatives