feat: Add subuser impersonation to client

[CaptainYarb]

Fixes #599

Checklist

• I acknowledge that all my contributions will be made under the project's license
• 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:

Sendgrid API has an (as yet) undocumented feature allowing parent accounts to impersonate
subusers by including an HTTP header "On-Behalf-Of" in each API request.
This commit enables setting the value for impersonation when creating the
Sendgrid API Client instance.

[thinkingserious]

@Blazedd

We have not been able to merge your Pull Request, but because you are awesome - we wanted to make sure you could still get a SendGrid Hacktoberfest shirt.

Please go fill out our swag form before Nov 5th and we will send the shirt! (We know that you might have tried this before and it didn’t work, sorry about that!)

You have till Nov 5th to fill out this form in order to get the Hacktoberfest shirt!

Thank you for contributing during Hacktoberfest! We hope to see you in the repos soon! Just so you know, we always give away a SendGrid shirt for your first ever non-Hacktoberfest PR that gets merged.

[childish-sambino]

License checkbox needs to be ticked in order to merge.

[CaptainYarb]

Checked. So sorry about the delay on this! 😅