Add an option for removing imported car files for offline deals

[web3-providers]

I am aware that there might be some use cases where the provider would like to keep the .car file that they are importing for later or even indefinitely, but most of us probably would like to have the option to get rid of it immediately after it has been imported/added to a sector or when the sector it has been added to has been finalized.

Personally, I would like to have at least an option to remove it early. That would save us time in implementing our own housekeeping scripts relying on constant polling of data via CLI/APIs, and it would lead to a single operation initiated by boostd.

At first glance, an option for the boostd import-data command looks most suitable, for example:

boostd import-data --remove-early $CID $FILE for removing it immediately after it is no longer needed (i.e., passed to the sealing workers),

or

boostd import-data --remove-late $CID $FILE for removing it after the sector has been published or proven.