[FEATURE] Add specs for `replication` namespace

[nhtruong]

Part of Adding missing API specs issue

• /_plugins/_replication/_autofollow: ["POST","DELETE"]
• /_plugins/_replication/autofollow_stats: ["GET"]
• /_plugins/_replication/follower_stats: ["GET"]
• /_plugins/_replication/leader_stats: ["GET"]
• /_plugins/_replication/{index}/_pause: ["POST"]
• /_plugins/_replication/{index}/_resume: ["POST"]
• /_plugins/_replication/{index}/_start: ["PUT"]
• /_plugins/_replication/{index}/_status: ["GET"]
• /_plugins/_replication/{index}/_stop: ["POST"]
• /_plugins/_replication/{index}/_update: ["PUT"]

[Tokesh]

Hello! I would like to work on this issue! :)