REST API: Add support for comments of password-protected posts.

Core requires the post password to view and create comments on password protected posts, so we must support a “password” param on the comments endpoint when fetch comments for a specific post and creating a comment on a password protected post.

Props flixos90, jnylen0.
Fixes #38692.
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git-svn-id: https://core.svn.wordpress.org/trunk@39289 1a063a9b-81f0-0310-95a4-ce76da25c4cd