feat: [DSM-98] Engine deletion

[michael-weigelt]

This PR introduces subnet deletion, but restricts the feature to CloudEngines.

For the registry, deleting a subnet means to:

• Remove its subnet ID from the key subnet_list.
• Remove its subnet record.
• Remove all routing table shards that its subnet ID maps to.
• Remove the catch up package.
• Remove the subnet public key.

Consumers of subnet_list, the subnet record and the routing table assume live subnets, so the existing interfaces remain semantically unchanged.
Consumers that must take deleted subnets into account do so via old registry versions. One such consumer will be the orchestrator, in a follow-up PR.

[alin-at-dfinity]

Since I'm not being asked to review this, just one random question. (o:

[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

Updated unreleased_changelog.md

No breaking changes

No data migration needed

Security review not needed because the feature is limited to CloudEngines. We'll request an in-depth review before the feature is opened to any subnet.