By default, a post most contain any of the requested terms to be included in the response. This commit adds a new `operator` property that can be set to `AND` to require a post to contain all of the requested terms.
For example, `/wp/v2/posts?tags[terms]=1,2,3&tags[operator]=AND` will return posts that have tags with the ids of 1, 2, and 3.
Props dlh, earnjam, Clorith, jnylen0, sebbb.
Fixes#41287.
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git-svn-id: http://core.svn.wordpress.org/trunk@50635 1a063a9b-81f0-0310-95a4-ce76da25c4cd