Add service migrations

[HypeMC]

A continuation of #524.

Unlike the previous PR, which attempted to support both migrations as services and regular migrations, this one introduces a new enable_service_migrations option, which greatly simplifies the overall implementation.

When enabled, Doctrine will fetch migrations from Symfony’s service container.

To use migrations as services, they need to be autoloadable. IMO, the best approach is to place them inside your src directory, for example:

doctrine_migrations:
+   enable_service_migrations: true
    migrations_paths:
-       'DoctrineMigrations': '%kernel.project_dir%/migrations'
+       'App\Migrations': '%kernel.project_dir%/src/Migrations'

[greg0ire]

Please add documentation in docs/index.rst

[HypeMC]

@greg0ire I've updated the documentation. Please let me know if it's ok.

[greg0ire]

Thanks @HypeMC !