Add missing config schema for new configurations provided by the module

Problem/Motivation

The module introduces new configs to the views, but doesn't declare a config schema for it.

Steps to reproduce

Install the Configuration Inspector module and run the following on an affected views config.

drush config:inspect views.view.events --detail

-------------------------------------------------------------------------------------------------------------------------------------------- ---------------- ------------- ------
  Key                                                                                                                                          Status           Validatable   Data
 -------------------------------------------------------------------------------------------------------------------------------------------- ---------------- ------------- ------
  views.view.events:display.default.display_options.exposed_form.options.bef.filter.combine.options_show_only_used                             missing schema
  views.view.events:display.default.display_options.exposed_form.options.bef.filter.combine.options_show_only_used_filtered                    missing schema
  views.view.events:display.default.display_options.exposed_form.options.bef.filter.combine.options_hide_when_empty                            missing schema

Proposed resolution

Add config schema.

Remaining tasks

Provide issue fork/patch.

User interface changes

N/A