Allow to work with non-text fields

Problem/Motivation

When I tried to use this process plugin for the uid field (i.e., node author user), this plugin could not find the existing value, because it looked at 'value' key of the field value.

The uid field is an entity reference, so it uses 'target_id' key.

Proposed resolution

Call getString() to get the string value of the field, instead of assuming that it is inside value key.
See the MR.

Remaining tasks

Code review, user testing.