Add method getConfigurationValue on BlockPluginTrait

Problem/Motivation

Making a custom block in a development site I noticed that there is a method to set configurations value for a block form and you can use it as:
$this->setConfigurationValue('config', $form_state->getValue('config'));
This call sets a value on configurations array but there is no version to get a value from that array.

Proposed resolution

Adds a method on BlockPluginTrait trait like getConfigurationValue to get a value from configurations array.