Add ability to impersonate subuser

[danilospa]

Resolves #270

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• It enables a impersonate_subuser parameter on client initialize function. When it's filled, the header On-Behalf-Of is added.

[SendGridDX]

All committers have signed the CLA.

[danilospa]

The following tests are breaking because corresponding files do not exist:
test_docker_compose_exists, test_docker_exists, test_codeclimate_exists.
May I remove these tests on this same PR?

[jagthedrummer]

I'd love to see this get merged.

[thinkingserious]

Thanks for the vote @jagthedrummer!

This PR is on our backlog to merge and your vote helps give it priority.

[clee]

LGTM - this PR is almost identical to #272, but I think the tests in this one are slightly better. I'd vote we merge this one.

[thinkingserious]

Hello @danilospa,

Thanks again for the PR!

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[Shadowssong]

Hi @thinkingserious do you know when the next release plans to happen? I would like to use this feature but its only on master and has not been released yet (as far as I can tell).