refactor(governance): Use slightly more authoritative source of Subnet Rental canister API types.

[daniel-wong-dfinity-org]

Related Work

In PR 6722, I tried sourcing the definition of these types from the the true authoritative source, the subnet-rental-canister repo. Unfortunately, that created various headaches. The headache that convinced me to abandon the aforementioned PR was how it increased the size of the NNS Governance WASM. The increase was not super big, but since WASM size is a more or less scarce resource, I didn't want to eat into our remaining headroom. Therefore, I went for this less ambitious PR instead.

References

Closes NNS1-3965.

[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

No canister behavior changes.