feat(DiscountCodes): add `setKey` action

[ragafus]

Summary

The key field of a DiscountCode will be modified in the scope of https://commercetools.atlassian.net/browse/PRC-3476 https://github.com/commercetools/merchant-center-frontend/pull/16925

This PR adds the setKey action to DiscountCodes.

Description

Docs https://docs.commercetools.com/api/projects/discountCodes#update-actions

Todo

• Tests
  • [ ] Unit
  • [ ] Integration
  • [ ] Acceptance
• [ ] Documentation
• [ ] Type label for the PR

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 87141bbad9be1fb2e234c57f85ef2aa5d2563a67

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | --------------------------- | ----- | | @commercetools/sync-actions | Minor |

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.75%. Comparing base (a4de02c) to head (87141bb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1892 +/- ## ======================================= Coverage 94.75% 94.75% ======================================= Files 147 147 Lines 5057 5057 Branches 1375 1375 ======================================= Hits 4792 4792 Misses 262 262 Partials 3 3 ```

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