Add get_forms() and get_landing_pages() API functions

[n7studios]

Summary

Adds get_forms() and get_landing_pages() API functions, as a convenience vs. having to call get_resources('forms') or get_resources('landing_pages').

Removes unnecessary timeout and encoding headers on requests for resources.

When fetching landing pages, exclude forms from the resultset.

Testing

• testGetForms: Tests that fetching forms works with expected response data.
• testGetLandingPages: Tests that fetching landing pages works with expected response data.

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)