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

[dereuromark]

Gap: no reset / removal API

static::$specificColumnTypes[] mutates a class-static array with no teardown. The bundled test registers 'my_custom_type' and
leaves it registered forever — fine for now because no other test uses that string, but:

• Real apps that register 'hstore' and later run a test asserting hstore is rejected will see the test flake
• Order-dependent tests are a long-term maintenance drag

Worth adding removeSpecificColumnType(string) and/or resetSpecificColumnTypes() — five lines plus a tearDown clearing them.

Gap: the test doesn't live where it ought to

testAddColumnWithSpecificColumnType is in tests/TestCase/Db/Table/TableTest.php, which makes sense as a general-machinery
test. But there's no per-adapter integration test proving the registered type actually round-trips through create() → DDL →
schema reflection. Given Cake's dialect fallback (strtoupper($type) for unknown types emits HSTORE correctly), a real Postgres
test adding 'hstore' and asserting the table was created with an hstore column would be ~10 extra lines and catch regressions
if the dialect ever changes that fallback.