Added custom attributes to certificate

[ccs24]

No description provided.

[ccs24]

Sorry I have missed description. When we issue a new certificate for our participant, we have to add some additional information that depends on the course, e.g. course start date and end date, number of hours, trainer's name, etc. We have this data defined in Accredible as custom attributes, but they can't currently be filled in the Moodle module.

That's why I added additional functionality that makes this possible:

1. I added a field in the database to store this additional information in json format
2. When downloading the list of attributes from Accredible, I omitted the restriction that the data must be text
3. I modified the form for editing certificate settings to display this data as text fields (I added the temporary slugify function to manage data)
4. When saving data from the form, data from these additional fields are saved in the json format
5. To make them appear in the issued certificate, I modified the function that retrieves dynamic data for certificates and added the function of adding information from additional database fields.

I hope that the proposed changes will be useful to more users of this module (and Accredible)