Add CPI periodicity data to ETL process

[jt-simon]

The cu.periodicity file is currently not part of the ETL process.

The file is located here: https://download.bls.gov/pub/time.series/cu/cu.periodicity

More info on the specifics of the file: https://download.bls.gov/pub/time.series/cu/cu.txt

To add this, need to:

1. Add an entry for the file information in EconDataLens.Etl.appsettings.example.json
2. Add an entity to EconDataLens.Core.Entities.Cpi
3. Add a method to the EconDataLens.Core.Interfaces.ICpiDataFileParser interface and the implementation
4. Add a method to the EconDataLens.Core.Interfaces.ICpiIngestionRepository interface and implementation
5. Add a method to the EconDataLens.Core.Interfaces.ICpiIngestionService interface and implementation
6. Add the entity to the EconDataLens.Data.EconDataLensDbContext and set up foreign keys on the other entities, constraints, etc.
7. Ensure the implementation EconDataLens.Services.CpiEtlService imports the cu.periodicity file.
8. Add a parser test for the new parser method in the EconDataLens.Tests project.