[Feature]: Introduce CentralBundle with local overrides

[kaurov]

Feature description

Usually, Pimcore-Partners-Agencies implement and sell MANY websites.
We usually use some central bundle with core Model and Services to share among all created websites.
Such bundle is usually connected as vendor/bundle in composer.json
In some special cases, it can be connected as GitSubmodule.

It makes us easier to:

• update/bugfix all created website in 1 place
• reuse company experience for the new project
• speed up new website creation process

What I propose to move to such CentralBundle in Demo application:

• the whole content of /src/ folder except car.*
• central twig templates. Local ones you can keep, but they will just include central ones -- to allow a developer to overwrite in the future if the project looks different.
• CSS files. With only 1 local CSS left
• Data-Object interface and definitions. Local Data-Object classes only extends central definitions, so admin can add more fields

[github-actions]

Thanks a lot for reporting the issue. We did not consider the issue as "Pimcore:Priority", "Pimcore:ToDo" or "Pimcore:Backlog", so we're not going to work on that anytime soon. Please create a pull request to fix the issue if this is a bug report. We'll then review it as quickly as possible. If you're interested in contributing a feature, please contact us first here before creating a pull request. We'll then decide whether we'd accept it or not. Thanks for your understanding.