fast_import: ensure storage import configuration remains stable through import

[VladLazar]

Currently, there's a few pageserver config fields which can be used to tune imports (see pageserver_api::config::TimelineImportConfig. They can impact the plan split.

An important property of imports is that the pageserver always produces the same plan given the same contents for the import bucket.

Hence, we should bake this config into the storage controller database entry for the import and use those values when generating the plan on import task restart.