Operation for exchanging the auth code for token

[sedovalx]

Front end should call backend operation to start exchange process. Add server method to exchange authorization code for access (and refresh) token. Input:

• authorization code
• server name

Output:

• access token
• refresh token (?)
• request info
• response info

Frontend should pass in an authorization code obtained on the previous step. The backend handler for the client request should

• get the server info by name
• build and call the token acquire uri

TOKEN_ENDPOINT?client_id=CLIENT_ID&client_secret=CLIENT_SECRET&grant_type=authorization_code&code=AUTHORIZATION_CODE&redirect_uri=CALLBACK_URL

• wait until the API answer on the callback uri #8
• response with token or error message and api request/response

Token info has the format:

{
    "access_token":"ACCESS_TOKEN",
    "token_type":"bearer",
    "expires_in":2592000,
    "refresh_token":"REFRESH_TOKEN",
    "scope":"read"
}

with possible additional fields.

[sedovalx]

Closing as double of #18