fix: SNS Gov canister should deserialize to proto Governance, not API Governance

[anchpop]

I think this was a bug in the code previously.

here is where we serialize Governance:

#[pre_upgrade]
fn canister_pre_upgrade() {
    log!(INFO, "Executing pre upgrade");

    let mut writer = BufferedStableMemWriter::new(STABLE_MEM_BUFFER_SIZE);

    governance() 
        .proto // This gets the `Governance` that's from `ic-sns-governance`
        .encode(&mut writer)
        .expect("Error. Couldn't serialize canister pre-upgrade.");

    writer.flush(); // or `drop(writer)`
    log!(INFO, "Completed pre upgrade");
}

And this is where we were deserializing it:

#[post_upgrade]
fn canister_post_upgrade() {
    log!(INFO, "Executing post upgrade");

    let reader = BufferedStableMemReader::new(STABLE_MEM_BUFFER_SIZE);

    match GovernanceProto::decode(reader) { // this is deserializing to the Governance that's in `ic-sns-governance-api`
        Err(err) => {
            // [...]
        }
        Ok(mut governance_proto) => {
            // Post-process GovernanceProto

            // TODO: Delete this once it's been released.
            populate_finalize_disbursement_timestamp_seconds(&mut governance_proto);

            canister_init_(governance_proto); // then in here we pass to canister_init_
            Ok(())
        }
    }
    .expect("Couldn't upgrade canister.");
    // [...]
}

canister_init_:

#[init]
fn canister_init_(init_payload: GovernanceProto) {
    let init_payload = sns_gov_pb::Governance::from(init_payload); // we convert the `ic-sns-governance-api` Governance to the `ic-sns-governance` Governance
    // [...]
}

This seems not quite right to me because we serialize one Governance (ic-sns-governance), but deserialize to the other Governance (ic-sns-governance-api) and then convert. Why not just deserialize to the ic-sns-governance Governance directly? This only works because the ic-sns-governance-api types still set prost derives, but I don't think we want to be protobuffing the ic-sns-governance-api types anyway.

(In fact, since we don't want to be protobuffing them, I actually remove their prost implementations in the next PR)

---

However, we should still initialize the canister using the API types. Doing this ensures that no proto types are part of our public candid interface. So the canister_init method has been restored and annotated with #[init], and takes the API type like canister_init_ did previously. Now canister_init_ takes the proto type, so it can be used by canister_post_upgrade and by canister_init after canister_init does the conversion.

---

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[github-actions]

If this pull request affects the behavior of any canister owned by
the Governance team, remember to update the corresponding
unreleased_changes.md file(s).

To acknowldge this reminder (and unblock the PR), dismiss this
code review by going to the bottom of the pull request page, and
supply one of the following reasons:

1. Done.

2. No canister behavior changes.

No canister behavior changes.