Support agent.features.include_tags_in_events - attach agent tags to collected events

## Summary

Part of the "Agent tags propagation" design (related issue: elastic/kibana#271551). Fleet agent tags are not currently forwarded into the events (logs, metrics, traces) collected by the agent, making them unusable for filtering/dashboarding. This issue tracks the agent-side change needed to close that gap.

## Proposed change

- Support the new policy field `agent.features.include_tags_in_events` (boolean, injected by Fleet Server, see fleet-server issue).
- When enabled, add a global processor that sets tags on outgoing events using the value from `agent.tags` (also injected by Fleet Server).
- `agent.features.include_tags_in_events` and `agent.tags` should also be passed through to Endpoint so it can apply the same tagging to its own data.

## Related

- Related issue: elastic/kibana#271551
- Kibana issue: elastic/kibana#276433
- Fleet Server issue: elastic/fleet-server#7321