Oracle OCI8 support for CI 4

It's been quite a while, but we are getting close to getting the Oracle driver into CI4.

If anyone can test it, please pull down the branch of the PR https://github.com/codeigniter4/CodeIgniter4/pull/2487 and test it.

Hi Kenji,

I have downloaded it and I am testing it against an Oracle Database 12c Release 2 Enterprise Edition (12.2.0.1.0).
It seems to work pretty well with the Doctrine integration (CLI ORM mapping, auto generation of entities, etc.)

Regards,
Bert