feat: comments of a discussion

[mabdh]

Is your feature request related to a problem? Please describe.
Comments is needed in discussion feature described in #47

Describe the solution you'd like

• Create comments API
• Add comments repository

Response Schema

{
  "id": 1,
  "body": "body here",
  "owner": {
		"id" : "1234-5678",
            "Email": "user@email.com"  
  }, // User
  "created_at" : timestamp,
  "updated_at": timestamp	
}

DB Schema

Column Name	Type	Example
id (PK)	serial	1
discussion_id (FK)	serial	1
body	text	This body could be written in markdown format
owner	uuid	1234-5678-9123
created_at	timestamp
updated_at	timestamp