feat(default_ad_api): add heratbeat api

[isamu-takagi]

Description

Add heartbeat API. See the documentation for details.

Related links

https://github.com/orgs/autowarefoundation/discussions/4561
autowarefoundation/autoware-documentation#536
https://autowarefoundation.github.io/autoware-documentation/main/design/autoware-interfaces/ad-api/features/heartbeat/

Tests performed

ros2 topic echo /api/system/heartbeat

Notes for reviewers

None

Interface changes

ROS Topic Changes

Topic Name	Type	Direction	Update Description
/api/system/heartbeat	autoware_adapi_v1_msgs/msgHeartbeat	Publisher	Heartbeat message.

ROS Parameter Changes

Effects on system behavior

None

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[yukkysaito]

LGTM