Add support for custom / site-specific attributes

Problem/Motivation

While this module supports several commonly-used link attributes out of the box, many sites (like ours) make use of additional link attributes that are sometimes site-specific / custom in nature. It would be ideal if the module opened up a way for us to add these additional attributes into the link dialog form in CKEditor.

It looks like prior versions of Linkit → may have had support for this, but the current version does not and seems to direct folks to this module for link attributes.

CKEditor 5 Plugin Pack → is sometimes suggested as an alternative to this module with custom attribute support, but it seems to work by exposing a toggle instead of a text field, and thus would not work well for more open-ended attributes (like title, class, id, etc.).

Steps to reproduce

N/A.

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

In addition to the six out-of-box attributes, users can also configure custom / site-specific attributes to show in the link dialog form in CKEditor per text format.

API changes

TBD

Data model changes

TBD