POC: produce Swagger documentation for subiquity's API

[ogayot]

Using python3-aiohttp-apispec, one can now document our API using @docs decorators in subiquity/common/apidef.py. I did a very simple example (without specifying the parameters and response schema).

The swagger.json file can be downloaded at /swagger.json

{
  "paths": {
    "/ssh": {
      "get": {
        "responses": {
          "200": {
            "description": "OK"
          }
        },
        "parameters": [],
        "summary": "Get SSH configuration",
        "description": "Return the current SSH configuration, including whether the SSH server is installed and enabled, and whether SSH keys are imported. ",
        "produces": [
          "application/json"
        ]
      },
      "post": {
        "responses": {
          "200": {
            "description": "OK"
          }
        },
        "parameters": [],
        "summary": "Configure the SSH server",
        "description": "Configure if the SSH server is installed and enabled.\nOne can also pass SSH keys to be added as authorized keys for the user.",
        "produces": [
          "application/json"
        ]
      }
    }
  },
  "info": {
    "title": "Subiquity API documentation",
    "version": "v1"
  },
  "swagger": "2.0"
}

It can then be fed to local tools or online tools such as https://editor.swagger.io/

Example on editor.swagger.io: