Extend the `system.dicts` helper for querying and modifying
`system.dicts` with an ability to use names other than "general".
We will use that in later commits to publish dictionaries for SSTable compression.
This "service" is a bag for code responsible for dictionary training,
created to unclutter main() from dictionary-specific logic.
It starts the RPC dictionary training loop when the relevant cluster feature is enabled,
pauses and unpauses it appropriately whenever relevant config or leadership
status are updated, and publishes new dictionaries whenever the training fiber produces them.
