celery_backend/config: Enable to set Sentry DSN per task type
Add a task_prerun
celery signal handler in order to set Sentry DSN based
on task name. The mapping between a task name and its DSN must be stored
in configuration under a task_sentry_dsn
key.
For this feature to work, no SWH_SENTRY_DSN
environment variable should
be defined as it overrides the sentry_dsn
value passed to init_sentry
function.
Related to swh/meta#4949 (closed).