[c7#121] For C7 Remote implement outbox pattern for external task completion

> Synced from https://github.com/bpm-crafters/process-engine-adapters-camunda-7/issues/121

### Scenario

* Library version: latest
* Description of your use case: In order to improve the confidence in TX implementation of the used client, we want make sure the adapter can use an outbox-pattern approach to complete Camunda jobs. More details here: https://medium.com/miragon/leveling-up-mastering-the-distributed-transaction-problem-in-zeebe-d4bbbca295d6

### Current Behaviour

Direct "TX after-commit" completion is used.

### Wanted Behaviour

Outbox pattern implementation based on Spring Data JPA (as one of the options)...

### Possible Workarounds

Manually implementation in every project.

Start by locating the adapter's current direct transaction-after-commit completion path, then read the linked outbox-pattern article and assess Spring Data JPA as the proposed option. Done means the adapter supports an outbox-based approach for completing Camunda jobs, with its transaction behavior covered by the project's tests.

Written by the indexing model from the issue text.