[reports-service] Introduce emails config

Summary:
We're going to use Postmark to send report emails. This diff introduces a simple config file format:

{
  "postmarkToken": "api key here",
  "senderEmail": "comm@example.com",
  "mailingGroups": {
    "inconsistencyReports": "mailgroup1@example.com",
    "mediaReports": "mailgroup2@example.com",
    "errorReports": "mailgroup3@example.com"
  }
}

The config will be loaded from CLI args or file - implemented in the next diff

Depends on D8865

Test Plan: Unit test, also tested with the next diff

Reviewers: michal, patryk, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8978