Zendesk Sell API and Contacts resource support

[Alex-Sh]

Added Sell API "Contacts" endpoint support
https://developers.getbase.com/docs/rest/reference/contacts

[Alex-Sh]

The only thing here is Zendesk Sell have different endpoint so HttpClient should be created like this:

$client = new HttpClient('', '', 'https', 'api.getbase.com');
$client->setAuth('oauth', ['token' => 'TOKEN']);

[AxelDeneu]

Is it possible to merge this PR ? Pretty useful if you use Sell API.

[ecoologic]

Hi,

thank you for your request,
are you still interested in a solution for this issue?

[ecoologic]

Closing due to inactivity. Re-open if needed.

[AxelDeneu]

Sorry, I missed your comment. I'm still interested in a solution for this issue.

[ecoologic]

Hi Alex, no worries. Thanks for your contribution.

[AxelDeneu]

Thank you too!
If needed, I can contribute on this PR, I don't know if @Alex-Sh is still interested into but I am. So if I can help in any kind, tell me.

[token-cjg]

Thank you for your contribution @Alex-Sh, I have accepted your suggestion in this PR: #537.