feat: add diagnostics api

[isamu-takagi]

Description

Add diagnostics API. See the generated document for details.

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[github-actions]

Documentation URL: https://autowarefoundation.github.io/autoware-documentation/pr-539/
Modified URLs:

• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/features/diagnostics/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/list/api/system/diagnostics/status/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/list/api/system/diagnostics/struct/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/list/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/types/autoware_adapi_v1_msgs/msg/DiagGraphStatus/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/types/autoware_adapi_v1_msgs/msg/DiagGraphStruct/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/types/autoware_adapi_v1_msgs/msg/DiagLinkStruct/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/types/autoware_adapi_v1_msgs/msg/DiagNodeStatus/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/types/autoware_adapi_v1_msgs/msg/DiagNodeStruct/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/types/
• https://autowarefoundation.github.io/autoware-documentation/pr-539/design/autoware-interfaces/ad-api/use-cases/system-monitoring/

[yukkysaito]

LGTM