Provide optional-field accessor helpers in score::json (GetAttribute usability)

### What

When parsing JSON configuration with `score::json`, consumers cannot cleanly express the common pattern "use the default if the key is missing, fail fast if the key exists but has the wrong type", because:

1. `GetAttribute(obj, key)` does not accept a raw `const Object&` (only `reference_wrapper` or `Result<...>`), forcing callers to wrap with `std::cref()` or fall back to manual `find -> second.As()`.
2. `Result::value_or(default)` swallows type errors together with "key not found", so a mistyped field silently falls back to the default instead of producing a clear error.

While implementing the TimeSlave JSON config parser (https://github.com/eclipse-score/time/pull/158) I had to duplicate the same ~10-line helper wrapper. Other components parsing JSON configs will likely hit the same issue.

### Acceptance Criteria (DoD)

The score::json documentation (README or header docs) is updated to include:
An example showing GetAttribute(std::cref(obj), key) as the standard way to access object fields, clarifying that std::cref/std::ref is the intended calling convention.
An example of the "default on missing key, fail fast on type error" pattern, demonstrating how to distinguish Error::kKeyNotFound from other errors without using value_or.

### How

_No response_