| Elektra 0.11.0

Use Case: `Key` clear

# Summary

- Scope: `libelektra-core`
- Level: Developer Goal
- Actors: Core, Caller
- Brief: Clearing contents of a `Key`
- Status: Implemented

# Scenarios

- Precondition:
  - `Key` has been created
- Main success scenario:
  - Caller requests the fields of a `Key` should get cleard
  - Name, value and metadata of the `Key` are empty after clearing
- Alternative scenario:
- Error scenario:
  - `Key` has name, value or metadata marked as read-only (e.g., because `Key` is part of ordered collection like `KeySet`)
  - Caller requests to clear `Key`
  - Core returns error and name, value and metadata of `Key` remain unchanged
- Postcondition:
- Non-functional Constraints: