Allow specific column type

[Erwane]

Allow addition of specific adapter column type.

[dereuromark]

Refs #1068

Small issue: SqliteAdapter silently ignores the registration

The PR puts $specificColumnTypes + addSpecificColumnType() on AbstractAdapter, implying universal support. But I checked all four adapters' getColumnTypes():

• MysqlAdapter.php:1335 — array_merge(parent::getColumnTypes(), static::$specificColumnTypes) ✓
• PostgresAdapter.php:1172 — same pattern ✓
• SqlserverAdapter.php:1076 — same pattern ✓
• SqliteAdapter.php:1664 — return array_keys(static::$supportedColumnTypes) — different property, no merge

So SqliteAdapter::addSpecificColumnType('foo') writes to AbstractAdapter::$specificColumnTypes (via LSB, since Sqlite doesn't redeclare it) and then isValidColumnType() never consults that list. Silent no-op on Sqlite. Either fix Sqlite to also merge, or keep the API Postgres/MySQL/SqlServer-only and document that.

[dereuromark]

Should also target 5.next