feat: Add close method to Client

[tsuyoshizawa]

Client class closes http client by finalize method.
However, the method is not promised to be called.

The class should have close method to call closing http client expressly.
In additional, we shouldn't call finalize method expressly if we want to close the client.

Closeable interface supports try-with-resources Statement.
https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Thank you @tsuyoshizawa!

Would you mind signing our CLA so that we may merge your work? It's just a quick web form.

With Best Regards,

Elmer

[tsuyoshizawa]

@thinkingserious I signed the CLA.

[andy-trimble]

LGTM

[mbernier]

@tsuyoshizawa

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.

[thinkingserious]

Hello @tsuyoshizawa,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX