[Filebeat] Add health status reporting to Redis input

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

The Redis 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 by reading the Redis input and the input/v2 context.UpdateStatus() API, then compare the CEL input implementation at x-pack/filebeat/input/cel/input.go. Clarify the meanings of the five listed states and the required context for non-HEALTHY states. Done means status changes are reported only when they change and all agreed states are covered.

Written by the indexing model from the issue text.