Support dim table loading without server restart

Currently, if we enable `dimTable` feature, we need to restart the server to get the table loaded. Instead, the table should be loaded by the segment reload API.

Start by tracing the segment reload API and the existing dimTable loading path in the Pinot source. Determine how reload requests currently handle dimTable, then verify that enabling the feature allows its table to load through the API without restarting the server.

Written by the indexing model from the issue text.