Add a new function get_page_by_guid

[killua99]

We have two different functions to get a post base from another field rather than ID get_page_by_path and get_page_by_title. When working with RESTful API working with unique identifier could be more handy than auto incremental or a path that could change.

I propose to build a function named get_page_by_guid and help developers to search a Post more accurate with guid.

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New function get_post_by_guid

Trac ticket: https://core.trac.wordpress.org/ticket/49708