Add old_la_codes (3 digit) to the wd_pcon_lad_la_rgn_ctry lookup and fetch_las()

[cjrace]

Is your feature request related to a problem? Please describe.

There's still quite a few places in DfE where we use and publish the 3 digit codes for LAs, we should add those in to save needing to source from elsewhere.

Describe the solution you'd like

Extra column joined to wd_pcon_lad_la_rgn_ctry before saving. Source for this might be different to get through an API or nice reliable connection. Worth checking if we can use GIAS / giasr as a source. Might need to find a lookup from somewhere internally or the screener repo and use that as an additional source - https://github.com/dfe-analytical-services/dfe-published-data-qa/blob/main/data/las.csv.

Describe alternatives you've considered

Could create a separate LA file similar to what is in the screener repo currently, might be useful, though would be a question of where to generate the fetch_las() function from, and how to manage duplication between them.

Additional context

Nothing to add.