[FEATURE] Add specs for `reports` namespace

[nhtruong]

Part of Adding missing API specs issue

• /_opendistro/_reports/_local/stats: ["GET"]
• /_opendistro/_reports/definition: ["POST"]
• /_opendistro/_reports/definition/{reportDefinitionId}: ["GET","PUT","DELETE"]
• /_opendistro/_reports/definitions: ["GET"]
• /_opendistro/_reports/instance/{reportInstanceId}: ["GET","POST"]
• /_opendistro/_reports/instances: ["GET"]
• /_opendistro/_reports/on_demand: ["PUT"]
• /_opendistro/_reports/on_demand/{reportDefinitionId}: ["POST"]

---

• /_plugins/_reports/_local/stats: ["GET"]
• /_plugins/_reports/definition: ["POST"]
• /_plugins/_reports/definition/{reportDefinitionId}: ["GET","PUT","DELETE"]
• /_plugins/_reports/definitions: ["GET"]
• /_plugins/_reports/instance/{reportInstanceId}: ["GET","POST"]
• /_plugins/_reports/instances: ["GET"]
• /_plugins/_reports/on_demand: ["PUT"]
• /_plugins/_reports/on_demand/{reportDefinitionId}: ["POST"]