formly type: add multicheckbox type

[Garfield-fr]

This type can be used on an array type of the json schema to have multiple checkboxes.

Co-Authored-by: Bertrand Zuchuat bertrand.zuchuat@rero.ch

Code review check list

• [ ] Commit message template compliance.
• [ ] Commit message without typos.
• [ ] File names.
• [ ] Functions names.
• [ ] Functions docstrings.
• [ ] Unnecessary commited files?
• [ ] Extracted translations?