[reports-service] Add functions to send e-mails

Summary:
This diff adds a Postmark client and two functions: One to compose an e-mail object and another to send all the emails in batch

Depends on D8986

Test Plan: Test plan in child diff D898 where these functions are called

Reviewers: michal, patryk, jon, tomek

Reviewed By: michal, tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8987