Devel generate support

Problem/Motivation

To support devel_generate and similar modules that use default values are not supported.

Steps to reproduce

Create content with field year
Install and enable devel_generate
Generate content
Field year is empty.

Proposed resolution

Implement `generateSampleValue()` method for field item.

Remaining tasks

Review.

User interface changes

No.

API changes

No.

Data model changes

No.