Define schema for the entity_log.configuration file

Problem/Motivation

When the module is configured, it creates a configuration object that stores the configured entities in the form of:

[entity_type]:
  [bundle]:
    fields:
      field1: 0
      field2: 0
      ..
      fieldN: 0

The schema for this should be defined in the config/schema/entity_log.configuration.yml file.

Steps to reproduce

N/A

Proposed resolution

Define the schema file.