Accept named meta_query clauses

[felipeelia]

Description of the Change

This PR introduces the ability to sort queries by named clauses in the meta_query argument and also adds support to sort by different meta types.

Closes #3369
Closes #3326

How to test the Change

Run the following query and see it working instead of throwing the No mapping found for [named_meta_query_key] in order to sort on error.

$args = array(
	'ep_integrate' => true,
	'fields'       => 'ids',
	'meta_query'   => [
		'named_clause' => [
			'key'     => 'test_key',
			'compare' => 'EXISTS',
		],
	],
	'orderby'      => 'named_clause',
);
$query = new \WP_Query( $args );

Changelog Entry

Added - Sort meta queries by named clauses and sort by different meta types.

Credits

Props @felipeelia @selim13

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.