Support for Link Attributes widget

Problem/Motivation

Add support for attributes on a link element. Now its impossible to select the attributes from a link as a slot or prop.
Link attributes also supports adding custom attributes and this is something that should be supported also.

Steps to reproduce

• Add the module to your website
• Set a link field to use the attributes widget on a content type
• On the display of a content type you can set the pattern to have for example a button pattern
• See that you can only select the base field props and not the attributes prop

Proposed resolution

Add a source plugin for the link attributes

Remaining tasks

Discuss for best implementation
Add MR
Review MR

User interface changes

Extra props for the link attributes

API changes

None

Data model changes

Extra source for the attributes