Add an entrypoint to use a custom ReferenceRepository

[VincentLanglet]

Hi !

I'd like to use a custom ReferenceRepository (https://github.com/doctrine/data-fixtures/blob/1.6.x/src/ReferenceRepository.php),

Currently:

• The ReferenceRepository is always set inside the AbstractExecutor, even if the fixture already has a referenceRepository
  https://github.com/doctrine/data-fixtures/blob/1.6.x/src/Executor/AbstractExecutor.php#L119-L121
  it could be nice to only set the referenceRepository if some method hasReferenceRepository() return false.

• The ReferenceRepository of the AbstractExecutor is hardcoded
  https://github.com/doctrine/data-fixtures/blob/1.6.x/src/Executor/AbstractExecutor.php#L47
  It could be useful to have the referenceRepository as optional argument...

• The Executor is hardcoded in the command https://github.com/doctrine/DoctrineFixturesBundle/blob/3.5.x/Command/LoadDataFixturesDoctrineCommand.php#L154

I see two way/improvements to solve my issue:

1. Solving Allow to inject any loader into LoadDataFixturesDoctrineCommand #380 to allow using custom fixtures loaders, then my FixtureLoader will do

$fixture = new class();
$fixture->setReferenceRepository($myCustomRepository);

AND I'll need to add a check in the AbstractExecutor (https://github.com/doctrine/data-fixtures/blob/1.6.x/src/Executor/AbstractExecutor.php#L119) to not set the ReferenceRepository if one is already set (But how do I know this since currently no method is exposed ?)

2. Solving Have the ability to chose any executor in LoadDataFixturesDoctrineCommand #364 by allowing to use ExecutorFactories in order to use a custom executor. My factory will just do:

$executor = new ORMExecutor($em, $purger);
$executor->setReferenceRepository($myCustomRepository);

What do you think of this problem ? Any suggestion/preferred solution @greg0ire @derrabus ?
Thanks