63946: Add "resolvable" routes for the REST API

[rmccue]

Adds the ability to register just-in-time resolvable routes for the REST API.

When calling register_rest_route(), you can now pass a function instead of the options for the route directly:

// Before:
register_rest_route( 'example/v1', '/ping', [
	'callback' => 'ping_callback',
	'permissions_callback' => '__return_true',
	'args' => [
		'foo' => [
			'required' => true,
			'type' => 'string',
		],
	],
	'schema' => $this->get_schema(),
] );

// After:
register_rest_route( 'example/v1', '/ping', fn () => [
	'callback' => 'ping_callback',
	'permissions_callback' => '__return_true',
	'args' => [
		'foo' => [
			'required' => true,
			'type' => 'string',
		],
	],
	'schema' => $this->get_schema(),
] );

This has the benefit that any more expensive operations (translations, args-to-schema building, etc) are only run for matched routes, rather than all of them. But, routing is still possible without this (including listing all available namespaces).

Trac ticket: https://core.trac.wordpress.org/ticket/63946

See also #10080

---

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[rmccue]

(cc @prettyboymp @kraftbj for feedback too, as an alternative to #10080)

[swissspidy]

Love the ergonomics of this approach 👍

[kraftbj]

On first glance, these two approaches seem complementary rather than competing, as they tackle 63946 at different layers of the stack.

#10080 defers at the namespace boundary: register_rest_route() never runs for a namespace that isn't matched, which also sidesteps whatever REST-only setup plugins put alongside registration — controller instantiation, register_rest_field, schema construction, boot-time translations, etc.

This one defers at the options boundary: register_rest_route() still runs for every route, but the options array (schema building, args-to-schema, translated strings inside the definition) is only materialized when that specific route is dispatched. Finer-grained & great for routes with heavy schemas, but inside the register call, not around it.

The two stack nicely. We could register a lazy namespace via #10080, and in the action that loads it, use resolvable options for any routes with expensive schemas that might still not be the one hit. Skip the namespace entirely, then if the namespace is hit, skip option construction for non-dispatched routes, and then only fully resolve the actually matched route.

tl;dr: :why-not-both.gif: