[FEATURE] Add specs for observability namespace

[nhtruong]

Part of Adding missing API specs issue

• /_plugins/_observability/_local/stats: ["GET"]
• /_plugins/_observability/object: ["GET","POST","DELETE"]
• /_plugins/_observability/object/{objectId}: ["GET","PUT","DELETE"]

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• /_plugins/_query/_datasources: ["GET","PUT","POST"]
• /_plugins/_query/_datasources/{dataSourceName}: ["GET","DELETE"]
• /_plugins/_query/settings: ["PUT"]

[aabeshov]

Hello , I'd like to work on this issue if it's still relevant.

[dblock]

It sure is! Please do contribute.