A serial/primary key field can not be added to an existing table for some databases

Problem/Motivation

- DBTNG does not support adding a serial field to an existing table, even if all the supported DB engines can do that just fine
- DBTNG does not handle primary keys properly when adding a new field to an existing table. \Drupal\Core\Database\Schema::changeField() documents that primary keys have to be handled manually by the calling code, but \Drupal\Core\Database\Schema::addField() documents that it handles primary keys by itself.

Proposed resolution

Fix DBTNG to support adding a serial field to an existing table.

Fix implementations of \Drupal\Core\Database\Schema::addField() to handle primary key changes automatically.

Remaining tasks

Review.

User interface changes

Nope.

API changes

Nope.

Data model changes

Nope.