[5.8] Fix the remaining issues with registering custom Doctrine types.

[jensdenies]

This PR removes the TinyInteger type from the core and allows the user to register their own custom Doctrine DBAL types. No connection wil be established by only instantiating the builder class, only as soon as you call the registerCustomDoctrineType method. Therefore it won't cause issues like #28282 anymore.

[kevindb]

If support for tinyint is being removed from core Laravel, does the documentation ned to be updated to remove references to tinyIncrements , tinyInteger, and unsignedTinyInteger?
https://laravel.com/docs/5.8/migrations#creating-columns

[driesvints]

@kevindb tinyint support wasn't removed in general, just the MySQL override introduced by #28214 because of the bug from #28282.