D7 field migration field_type should be extendable

Problem/Motivation

The Drupal 6 field migration (core/modules/field/migration_templates/d6_field.yml) uses the "field_type" plugin (class: \Drupal\field\Plugin\migrate\process\d6\FieldType) to provide an extendable static map, to map D6 field types to D8 field types.
The field_type plugin tries to get a D8 value by calling getFieldType() on the appropriate CckFieldPlugin.

The Drupal 7 field migration variant core/modules/field/migration_templates/d7_field.yml just uses a static map, and doesn't allow to use the CckFieldPlugin.
The extendability wasn't taken into account when writing the D7 field migrations (#2416765: Migrate Drupal 7 Field/Instance/View mode settings).

Proposed resolution

Use the same principle in D7 as is used in D6. Though the CckFieldPlugin functionality seems to indicate it is only useful for D6, given the name, it should also be used to migrate (contrib) field types from D7.
Related to this is #2631736: Cckfield Plugins must distinguish core versions, which allows to make a distinction between D6 & D7 CckFieldPlugins.

Remaining tasks

* Write patch
* Write or update tests

User interface changes

none

API changes

D6 functionality will keep working, D7 functionality will be extended. Don't think that implies API changes.