Make field object support transitional call-based interface

This makes the Field object able to be called like a function, which
emulates the old interface that objects are expecting. When all the
existing objects are converted to fields, we can fix the rest of
the infrastructure and remove this transition helper.

Related to blueprint structured-object-fields

Change-Id: I605a1e22d443137d850d4aa262a8bf8b93feabd7
Merge "Make field object support transitional call-based interface"