Extend author field to other types than user entity reference

[colorfield]

The author field set from the bundle configuration could be from several types (user reference, text, ...). The current implementation assumes that the user reference type is the default.

Currently, the module has a limitation to the plain Author-Name attribute for the paper template.
Which opens the possibility for plain text field as source type.
However, user reference makes sense, because the paper template allows to set multiple attributes:

• Author-Name-First
• Author-Name-Last
• Author-Email
• Author-Workplace-Name

See https://ideas.repec.org/t/papertemplate.html

A minimal implementation needs at least a field validation during the field mapping on the bundle settings form, then coverage for other types if necessary.