REST API: Add plugin and block directory endpoints

[TimothyBlynJacobs]

Gutenberg is introducing REST API endpoints for installing blocks, and activating/deactivating plugins. This is a tracking ticket for handling the core merge of this feature.

​https://github.com/WordPress/gutenberg/pull/22454

Original Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/22454

This also introduces a change in plugins_api to not perform a trigger error if this is a json request since we don't want that to break the JSON output.

Trac ticket: https://core.trac.wordpress.org/ticket/50321

We'll also need a PR to update Gutenberg to use the wp/v2 namespace.

@TimothyBJacobs Can we keep everything experimental? Or are these APIs fully ready?

Thoughts on shipping experimental endpoints in core are [mixed](​https://make.wordpress.org/core/2020/06/14/shipping-experimental-endpoints-in-core-rest-api-meeting-summary-june-11-2020/) so I'm not really sure about shipping experimental endpoints in general.

For this endpoint in particular, I don't think it needs to be shipped as experimental.

Created a PR for updating the namespaces in Gutenberg: ​https://github.com/WordPress/gutenberg/pull/23528

I tested out the endpoints, and once I got this PR done ^{, I was able to test out using the block directory — it all worked as expected.}

[TimothyBlynJacobs]

In 48242:

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.

[SergeyBiryukov]

In 48333:

Tests: Correct hardcoded "3 months ago" assertion in tests/rest-api/rest-block-directory-controller.php.

Now that more than 3 months have passed since 2020-03-23, this was causing test failures.

Follow-up to [48242].

See #50321.

[justinahinon]

Related devnote here ​https://make.wordpress.org/core/2020/07/16/new-and-modified-rest-api-endpoints-in-wordpress-5-5.

[SergeyBiryukov]

In 48626:

Docs: Correct @since version for WP_REST_Plugins_Controller::get_item_schema().

Follow-up to [48242].

See #50321.

[SergeyBiryukov]

In 48627:

REST API: Use a consistent name for the textdomain schema property between plugins, themes, and block directory controllers.

Follow-up to [48242].

See #50321, #50776.

[SergeyBiryukov]

In 48628:

REST API: Use a consistent description for the textdomain schema property between plugins and themes controllers.

Follow-up to [48242], [48627].

Props audrasjb.
Fixes #50776. See #50321.