Abilities API: Normalize input from schema

[gziolo]

Bug fix extracted from WordPress/abilities-api#108.

Follow-up for https://core.trac.wordpress.org/ticket/64098.

Normalizes the input for the ability, applying the default value from the input schema when needed.

When no input is provided and the input schema is defined with a top-level default key, this method returns the value of that key. If the input schema does not define a default, or if the input schema is empty, this method returns null. If input is provided, it is returned as-is.

Trac ticket: https://core.trac.wordpress.org/ticket/64139

Bug fix extracted from ​https://github.com/WordPress/abilities-api/pull/108.

## The problem

Without this patch REST would require a weird empty ?input field given how the current controller works with input schema that defines specific shape, example:

'input_schema'        => array(
        'type'                 => 'object',
        'properties'           => array(
                'fields' => array(
                        'type'        => 'array',
                        'items'       => array(
                                'type' => 'string',
                                'enum' => $fields,
                        ),
                        'description' => __( 'Optional: Limit response to specific fields. If omitted, all fields are returned.' ),
                ),
        ),
        'additionalProperties' => false,
        'default'              => array(),
),

Working examples:

GET /run?input # All fields
GET /run?input[fields][]=name # Filtered

We want the REST API controller to infer default value from the input schema, so it's possible to call:

GET /run # All fields

## Solution

Normalizes the input for the ability, applying the default value from the input schema when needed.

When no input is provided and the input schema is defined with a top-level default key, this method returns the value of that key. If the input schema does not define a default, or if the input schema is empty, this method returns null. If input is provided, it is returned as-is.

[gziolo]

In 61047:

Abilities API: Normalize input from schema

Without this patch REST API would require a weird empty ?input field for optional input given how the current controller works with input schema when it defines the expected shape. This patch normalizes the input for the ability, applying the default value from the input schema when needed.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/10395.

Follow-up [61032], [61045].

Props gziolo, jorgefilipecosta, mukesh27.
Fixes #64139.

Trac ticket: https://core.trac.wordpress.org/ticket/64139

Addresses feedback raised by @jorgefilipecosta during syncing changes to Abilties API repository:

• ​https://github.com/WordPress/abilities-api/pull/126

Committed with https://core.trac.wordpress.org/changeset/61071.