[Filebeat] Add health status reporting to Kafka input

Update the Kafka input to report its health status, providing better visibility into the operational state for users.

The Kafka input should report its health status using the [`context.UpdateStatus()`](https://github.com/elastic/beats/blob/9190f776f5fb612a880cb013a9eec8a49fe17afd/filebeat/input/v2/input.go#L112) method. Status changes should only be reported when the status actually changes (don't repeatedly send HEALTHY if the status is already HEALTHY).

### Status Reporting
The input should report the following states:

- **STARTING**: TBD
- **CONFIGURING**: TBD
- **HEALTHY**: TBD
- **DEGRADED**: TBD
- **FAILED**: TBD

Include relevant context information with non-HEALTHY states:

- TBD

## References
See the CEL input implementation for reference:
https://github.com/elastic/beats/blob/6409d005a31e0147a40f0872794880b3df15a69c/x-pack/filebeat/input/cel/input.go#L111

Start with the Kafka input implementation and the context.UpdateStatus method in filebeat/input/v2/input.go, then compare the referenced CEL implementation in x-pack/filebeat/input/cel/input.go. The status meanings and non-HEALTHY context are still TBD; done requires defining those states and reporting changes only when the status differs.

Written by the indexing model from the issue text.