REST API: Introduce plugin management and block directory endpoints.

These endpoints facilitate the Block Directory Inserter feature in Gutenberg. Users can now install, activate, deactivate, and delete plugins over the REST API. The block directoryendpoint allows searching for available blocks from the WordPress.org block directory.

Props cklee, talldanwp, noisysocks, joen, soean, youknowriad, dufresnesteven, gziolo, dd32, tellyworth, ryelle, spacedmonkey, TimothyBlynJacobs.
Fixes #50321.


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