Job Pre/Post processing flexibility

- Currently, a VO can define 1 `JobExecutionCoordinator`, which happens just after the execution of the payload, before the upload of the result.
- A given VO might want to deal with output data in a different way, so the upload of the result should probably move inside the base `JobExecutionCoordinator`
- But a given VO might want to use specific operations from the base one, and redefine an operation in-between.

Currently, the system is not flexible enough to support that kind of use cases and should be revised.