[REF] Add DI for MailClient

[seaerchin]

Problem

In #369, the unit tests for MailClient was contorted using jest.resetCache to use a fresh copy of the env vars for testing. This is quite awkward and jarring, especially for engineers who do not know what resetCache does and why it's needed.

Solution

Rather than getting the key from the env var and using it directly, we should rewrite MailClient to use dependency injection and pass the POSTMAN_API_KEY as a parameter. This allows easier testing