Standardize EIA column names for harvesting

There are a number of column names which aren't currently standardized across the various EIA spreadsheet maps because they're preemptively dropped during the transform step. We need to stop doing that (see #509) and at the same time give them standard names that are part of the PUDL DB schema so they can be appropriately harvested.

A partial list of columns that need to be fixed. This also includes some other column name standardizations beyond the need to clean up harvested columns. This should probably be done in conjunction with #509 and #1250:

[ ] plants_eia860
- state -> plant_state
[ ] boiler_fuel_eia923
- naicscode -> code????_naics
- eia_sector -> sector_id_eia
- sector_name -> sector_name_eia
- fuel_unit -> fuel_units
[ ] fuel_receipts_costs_eia923
- state -> mine_state
- natural_gas_transport_code -> natural_gas_transportation_mode_code
[ ] generation_fuel_eia923
- naics_code -> ???_code_naics
- eia_sector -> sector_id_eia
- sector_name -> sector_name_eia
- fuel_unit -> fuel_units
[ ] generator_existing_eia860
- state -> plant_state
- county -> plant_county (and lots of other tables too?)
- turbines_num -> ???? wind turbines?
- startup_source_code_N -> startup_energy_source_code_N
[ ] generator_proposed_eia860
- state -> plant_state
- county -> plant_county???? (with lots of other tables too?)
- turbines_num -> something with wind???
- startup_source_code_N -> startup_energy_source_code_N
[ ] generator_retired_eia860
- state -> plant_state
- county -> plant_county???? (with lots of other tables too?)
- turbines_num -> something with wind???
- startup_source_code_N -> startup_energy_source_code_N

catalyst-cooperative / pudl

Standardize EIA column names for harvesting #1327