Let FieldInstanceConfigInterface extend ThirdPartySettingsInterface

Follow-up of #2224761: Add a generic way to add third party configuration on configuration entities and implement for field configuration.

Problem/Motivation

Since FieldInstanceConfig extends FieldConfigBase, and FieldConfigBase uses ThirdPartySettingsTrait, instances of FieldInstanceConfig can be expected to provide the ThirdPartySettings-related methods. (I'm still a bit inexperienced with this OOP stuff, so bear with me please...) If I understand correctly, it would make sense if FieldInstanceConfigInterface extends ThirdPartySettingsInterface, so other implementations of FieldInstanceConfigInterface must support third party settings too.

As a bonus, implementing ThirdPartySettingsInterface would allow variable type hinting on the FieldInstanceConfigInterface like this (thus enabling code completion in IDE's like PhpStorm):

use Drupal\field\Entity\FieldInstanceConfig;

/** @var \Drupal\field\FieldInstanceConfigInterface $config */ 
$config = FieldInstanceConfig::load('some.config.id');
$config->getThirdPartySetting('module', 'key', array()); // Typing "getT" in PhpStorm makes the IDE suggest this method.

Proposed resolution

Let FieldInstanceConfigInterface extend ThirdPartySettingsInterface.