Block Bindings: Prepare the ground to define fieldsList on the server.

[cbravobernal]

What?

Ignore this PR at this moment. I'm just experimenting if it is possible to add sources to the UI just by defining them on the server side. It will need a PR in Core to be landed.

Something like:

register_block_bindings_source(
			'bbe/now-date',
			array(
				'label'              => __( 'Current date', 'custom-bindings' ),
				'get_value_callback' => function ( array $source_args, $block_instance ) {
					return gmdate( 'Y-m-d H:i:s' );
				},
				'get_fields_list'    =>
					array(
						array(
							'id'    => 'date1',
							'label' => __( 'Date (Y-m-d)', 'custom-bindings' ),
							'value' => 'Y-m-d',
						),
						array(
							'id'    => 'date2',
							'label' => __( 'Date (d/m/Y)', 'custom-bindings' ),
							'value' => 'd/m/Y',
						),
					),
			)
		);

To return: