[FEATURE] Add specs for `dashboard` namespace

[nhtruong]

Part of Adding missing API specs issue

• [ ] /_opensearch_dashboards/_alias: ["GET","PUT"]
• [ ] /_opensearch_dashboards/_alias/{name}: ["GET","PUT","POST","HEAD"]
• [ ] /_opensearch_dashboards/_aliases: ["GET"]
• [ ] /_opensearch_dashboards/_aliases/{name}: ["PUT","POST"]
• [ ] /_opensearch_dashboards/_bulk: ["PUT","POST"]
• [ ] /_opensearch_dashboards/_mget: ["GET","POST"]
• [ ] /_opensearch_dashboards/_refresh: ["GET","POST"]
• [ ] /_opensearch_dashboards/_search: ["GET","POST"]
• [ ] /_opensearch_dashboards/_search/scroll: ["GET","POST","DELETE"]
• [ ] /_opensearch_dashboards/_search/scroll/{scroll_id}: ["GET","POST","DELETE"]
• [ ] /_opensearch_dashboards/_settings: ["PUT"]
• [ ] /_opensearch_dashboards/{index}: ["GET","PUT","HEAD"]
• [ ] /_opensearch_dashboards/{index}/_alias: ["GET","PUT","HEAD"]
• [ ] /_opensearch_dashboards/{index}/_alias/{name}: ["GET","PUT","POST","HEAD"]
• [ ] /_opensearch_dashboards/{index}/_aliases: ["PUT"]
• [ ] /_opensearch_dashboards/{index}/_aliases/{name}: ["PUT","POST"]
• [ ] /_opensearch_dashboards/{index}/_bulk: ["PUT","POST"]
• [ ] /_opensearch_dashboards/{index}/_create/{id}: ["PUT","POST"]
• [ ] /_opensearch_dashboards/{index}/_delete_by_query: ["POST"]
• [ ] /_opensearch_dashboards/{index}/_doc: ["POST"]
• [ ] /_opensearch_dashboards/{index}/_doc/{id}: ["GET","PUT","POST","DELETE","HEAD"]
• [ ] /_opensearch_dashboards/{index}/_mget: ["GET","POST"]
• [ ] /_opensearch_dashboards/{index}/_refresh: ["GET","POST"]
• [ ] /_opensearch_dashboards/{index}/_search: ["GET","POST"]
• [ ] /_opensearch_dashboards/{index}/_settings: ["PUT"]
• [ ] /_opensearch_dashboards/{index}/_update/{id}: ["POST"]

[dblock]

Are these public-facing, do we need documentation for them too? cc: @hdhalter