OAuth: Support tenant_name Parameter

[n7studios]

Summary

Adds support for specifying the tenant_name parameter, to issue unique OAuth tokens to multiple instances using the same Kit account and OAuth Client / App ID.

Testing

Manual end to end test completing OAuth flow with the same Kit account and OAuth Client / App ID confirms this working:

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)