refactor(governance): derive SelfDescribing for ManageNeuron nested types

[jasonz-dfinity]

Why

The From<T> for SelfDescribingValue impls for ManageNeuron nested types
were hand-written boilerplate that simply listed each field. The
SelfDescribing derive macro can generate these automatically.

What

• Added SelfDescribing derive to 12 ManageNeuron nested types
  (e.g., IncreaseDissolveDelay, Split, Spawn, Merge, etc.) and
  the NeuronIdOrSubaccount oneof in the protobuf generator config.
• Removed the corresponding hand-written From<T> for SelfDescribingValue
  impls (~130 lines of boilerplate).
• Updated test imports accordingly.

Testing

Existing unit tests cover the removed impls and verify the derive-generated
versions produce equivalent output.

[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

How to Satisfy This Automatic Review

1. Go to the bottom of the pull request page.

2. Look for where it says this bot is requesting changes.

3. Click the three dots to the right.

4. Select "Dismiss review".

5. In the text entry box, respond to each of the numbered items in the previous
   section, declare one of the following:

• Done.

• $REASON_WHY_NO_NEED. E.g. for unreleased_changelog.md, "No
  canister behavior changes.", or for item 2, "Existing APIs
  behave as before.".

Brief Guide to "Externally Visible" Changes

"Externally visible behavior change" is very often due to some NEW canister API.

Changes to EXISTING APIs are more likely to be "breaking".

If these changes are breaking, make sure that clients know how to migrate, how to
maintain their continuity of operations.

If your changes are behind a feature flag, then, do NOT add entrie(s) to
unreleased_changelog.md in this PR! But rather, add entrie(s) later, in the PR
that enables these changes in production.

Reference(s)

For a more comprehensive checklist, see here.

GOVERNANCE_CHECKLIST_REMINDER_DEDUP

N/A trivial change