AddToCalendar third party settings missing schema

Problem/Motivation

Split off from #3092765: AddToCalendar config does not match schema, plus some schema missing

The module uses third party settings to modify datetime field formatters. The third party settings needs schema.

This was split off from the other schema issue because it will require new test coverage and potentially its own update of post_update hook.

Proposed resolution

Add schema with test coverage.

Remaining tasks

User interface changes

nope

API changes

nope

Data model changes

Maybe an integer zero needing to get re-saved as a string zero. We'll see!