fix: Enforce single target network notes in NTB

[sergerad]

Context

Closes #1115.

The NTB only supports single target network notes and does not currently check for this when constructing NetworkNote from Note.

Changes

• Update NetworkNote to be an enum with single and multi target variants.
• Add SingleTargetNetworkNote and MultiTargetNetworkNote types.
• Use SingleTargetNetworkNote in NTB.

[bobbinth]

Looks good! Thank you. Overall, I think this approach works fine for now. We could make it more type-safe by introducing newtype for each type of notes. NetworkNote then could look something like:

pub enum NetworkNote {
    SingleTarget(SingleTargetNote),
    MultiTarget(MultiTargetNote),
}

pub struct SingleTargetNote(Note);
pub struct MultiTargetNote(Note);

And then most of the APIs would work with SingleTargetNote for now.

But I'll defer to @Mirko-von-Leipzig on this.

[bobbinth]

I would probably replace the TODO with a simple comment explaining that we handle only single-target accounts for now.

Also, we can probably re-write this as:

if let Some(prefix) = note.account_prefix() {
    let Some(account) = state.fetch_account(prefix).await? {
        account.add_note(note);
    }
}

Lastly, I would expand the comment and explain under which circumstances we may get a single-target note that doesn't have a target account.

[bobbinth]

Here too I would replace the TODO with a simple comment about not handling multi-target notes yet (mostly because enabling support for multi-target notes is a much bigger refactoring than a simple TODO here implies).

Also, I would expand the comment and explain why there could be non-existent network accounts.

[Mirko-von-Leipzig]

As mentioned; I would prefer a SingleTargetNetworkNote so that we're certain what we're working with everywhere.

We don't know what or how we're going to manage the other variant(s); I think adding a strong type will help us refactor in the future without worrying about screwing up.