Access to client as injectable service

Problem/Motivation

I want to be able to use the mailjet API client for calls not related to sending emails, such as list management. I usually expect an API module to expose the full API in a usable fashion.

Steps to reproduce

Try to access client property on MailjetApiHandler - you can't, it's protected (as it should be).

Proposed resolution

MailjetApiHandler handles two separate concerns:

1. Checking that everything is in place to use the API (library, configuration etc) and initialising the client
2. Sending emails.

Normally I would suggest that separating these concerns into two separate services would be the best approach, however I think that will cause backwards-compatibility issues... so unless there's appetite for a 2.x version I'd say that adding a getClient() method to the existing service would be enough.

Remaining tasks

1. Decide on an approach
2. Patch
3. Review
4. Commit

User interface changes

Unlikely.

API changes

Maybe, depends on approach.

Data model changes

None.