Add migration plugin to convert blocks to Simple Block configuration entities

Problem/Motivation

When migrating a site from D6 or D7 it would be useful to convert simple blocks to Simple Block entities.

Proposed resolution

Add a migration plugin that converts blocks in D6 and D7 to Simple Block entities.

Remaining tasks

Add a migration plugin that converts blocks in D6 and D7 to Simple Block entities.
Add test coverage.

User interface changes

n/a

API changes

TBD

Data model changes

TBD