There's a bunch of /storage_service/... endpoints that start compaction manager tasks and wait for it. Most of them have async peer in /tasks/... that start the very same task, but return to the caller with the task ID.
This patch moves those handlers' code from storage_service.cc to tasks.cc, next to the corresponding async peers, to keep handlers that need compaction_manager in one place.
That's preparation for more future changes. Later all those endpoints will stop using database from http_context and will capture the compaction_manager they need from main, like it was done in #20962 for /compaction_manager/... endpoints. Even "more later", the former and the latter blocks of endpoints will be registered and unregistered together, e.g. like database endpoints were collected in one reg/unreg sequence by #25674.
Part of http_context dependencies cleanup effort, no need to backport.
Closesscylladb/scylladb#26140
* https://github.com/scylladb/scylladb:
api: Move /storage_service/compact to tasks.cc
api: Move /storage_service/keyspace_upgrade_sstables to tasks.cc
api: Move /storage_service/keyspace_offstrategy_compaction to tasks.cc
api: Move /storage_service/keyspace_cleanup to tasks.cc
api: Move /storage_service/keyspace_compaction to tasks.cc
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