Add site option to API and CLI

[zachazar]

Description

Datadog now offers different sites in addition to EU (I imagine to load balance a bit). Here is the link to site docs. When your account uses a different site, the URL for ingesting logs is different. Here is a link to the API docs. On the API docs page if you select V1 and a different site, you'll see that the URL is different for ingesting logs.

This PR

• Adds a site option to allow you to change the datadog site (along with --site for cli). This option updates the url.
• Updates documentation
• Adds tests for new code

Checklist

• run npm run test
  

• tests and/or benchmarks are included

• documentation is changed or added

• commit message and code follows Code Of Conduct