Add a link field class, SgLinkField

[ndunsworth]

Look at possibly adding a type of SgField that is a link to another field.

Link fields don't need that much functionality just a call SgLinkField.link() that points to the field linked too.

Example use case:
Add a universal field called "name" to all entities that points to whichever whacked out field Shotgun decides to use for the name of the Entity. Like "code", "content", etc.

[ndunsworth]

Creating a link field may not be necessary for the "name" field example.

SgEntity.buildFields() or getitem may be a better place to just put code in that knows which field to grab.

It's a toss up for this example as one involves creating base class for all entity types that would need it while the other solution involves hard coding the base entity class.