Source Mailchimp: add oauth flow (java part)

[annalvova05]

Source Mailchimp: add oauth flow (java part)

https://mailchimp.com/developer/marketing/guides/access-user-data-oauth-2/

=========== JAVA PART ================

Approve access from the customer side using:
client_id

https://login.mailchimp.com/oauth2/authorize?response_type=code&client_id=1234567890&redirect_uri=https://airbyte.io/

Get redirect url with 'code':

https://airbyte.io/?code=33333333

Obtain token using

curl https://login.mailchimp.com/oauth2/token
-X POST
-H 'Content-Type: application/json'
-d '{
"client_id": "1234567890",
"client_secret": "2222222",
"grant_type": "authorization_code",
"code": "3333333",
"redirect_uri": "https://airbyte.io/"
}'

RESPONSE:
{
"access_token": "1q2w3e4r5t6y.",
"expires_in": "0",
"scope": null
}

OAuth access tokens do not expire.

Obtain server_prefix (dc) using

curl https://login.mailchimp.com/oauth2/metadata
-X GET
-H 'Authorization: Bearer <access_token>'

RESPONSE:
{
"dc": "us2",
"role": "admin",
"accountname": "Airbyte",
"user_id": 123,
"login": {
"email": "email@email.com",
"avatar": null,
"login_id": 12345,
"login_name": "email@email.com",
"login_email": "email@email.com"
},
"login_url": "https://login.mailchimp.com",
"api_endpoint": "https://us2.api.mailchimp.com"
}

============== PYTHON PART =====================

API requests using access_token and server_prefix (dc).

$ curl
-s https://{server_prefix}.api.mailchimp.com/3.0/campaigns
-H 'Authorization: Bearer <access_token>'

[antixar]

Duplicate
It'll be fixed with #6273