Unsanitized array keys not reported

Bug Description

phpcs only validates that the values of an array are sanitized
But it should check if array keys are sanitized too, since they can have any value and can be used for an attack in the same way as values are.

Minimal Code Snippet

$_POST['my-data'] = array( 'hello' => 'world' );

$my_data = array_map( 'sanitize_text_field', $_POST['my-data'] );

foreach ( $my_data as $key => $value ) {
    // use $key here
}

Same applies when using map_deep method too

Error Code

None

Environment

Tested Against develop branch?

• I have verified the issue still exists in the develop branch of WPCS.

Start by reproducing the minimal PHP snippet against the develop branch and trace the sniff that recognizes sanitized array values. Check the equivalent map_deep case as well. Done means unsanitized array keys are reported consistently with values, with regression coverage for both examples.