Add schema to bootstrap_toggle

Problem/Motivation

The module requires a config schema for the Bootstrap Toggle widget settings.

Steps to reproduce

1. Add a boolean field to an entity
2. In form display select "Bootstrap Toggle" as a widget
3. Open an entity with the above fiel in Edit mode.
4. Run Config Inspector and check the errors.

Proposed resolution

Provide a config schema for the config variables, e.g. bootstraptoggle_data_on, display label etc.

Remaining tasks

Provide a patch

User interface changes

None

API changes

None

Data model changes

None