Rework `POST /change-password` endpoint to make it simpler

[Syndesi]

The POST /change-password endpoint is currently using the following request body scheme:

{
  "type": "ActionChangePassword",
  "currentPassword": "1234",
  "newPassword": "9876",
  "data": {
    "<identifier>": "test@example.com"
  }
}

The user identifier at data.<identifier> is not really intuitive, especially because no other properties within data will be used anyways. Furthermore, as <identifier> depends on the instance's configuration, it is unknown without executing GET /instance-configuration as well, making the current implementation both unintuitive as well as hard to use.

The current solution was developed during #121 .

The proposed solution is as following:

{
  "type": "ActionChangePassword",
  "currentPassword": "1234",
  "newPassword": "9876",
  "uniqueUserIdentifier": "test@example.com"
}

---

Tasks

• [x] Change endpoint while keeping the old behavior available. Keep the old behavior behind a feature flag which is on by default, and will be disabled with the release of version 0.2.0.
• [x] Change the variable name user to uniqueUserIdentifier in the POST /token endpoint, keep the old logic available behind a feature flag as well.
• [x] Change the endpoint POST /register in a similar fashion.
• [x] Update unit tests. Mark old tests as deprecated.
• [x] Update feature tests.
• [x] Update documentation, especially graph & example generation.
• [x] Mark this as a breaking change in the change log.
• [x] Add new task in CI/CD for old, replaced feature tests.