DCOM-73: CodeGeneration using Doctrine\Common\Persistence\Mapping\ObjectMetadata

[doctrinebot]

Jira issue originally created by user @beberlei:

Currently we have tons of code in the ODM/ORMs regarding code generation that are ugly to extend and maintain. We should extract all them into a new component, for example named: doctrine-code-generator

• It should be feed by only instances of Doctrine\Common\Persistence\Mapping\ObjectMetadata into a twig template.
• A base template for an entity/document is shipped
• Maybe ORM/ODM specific child templates or twig traits are necessary to handle writing annotations. This could also be done by adding hooks into the code-generation somehow.
• Each entity of the user can provide its own template, for examples from a configurable directory "code_templates/Vendor.ProjectBundle.Entity.User.twig" to extend the base template and add own code.
• As a perspective we should aim for 5.4 generating a trait so that you can have your entity "class User { use UserBase; }"

[Majkl578]

Entity/repository codegen component in ORM has been dropped (doctrine/orm#6870) without replacement, this issue is no longer relevant.

For generic codegen, nikic/php-parser or nette/php-generator could be used.