[diracx-tasks] DEFAULT_RESULT_TTL should be configurable on a per task basis

At the moment, the result TTL is a parameter of the result backend
https://github.com/DIRACGrid/diracx/blob/main/diracx-tasks/src/diracx/tasks/plumbing/broker/result_backend.py

However, you may want to change that value on a per task bases.

- not restricted to specific tasks: can be applied on any task

## Example

Short tasks should have a shorter lifetime, but manually triggered tasks (FC dump) can be stored for few days.