Use business logic from {inkit} external package

[matt-dray]

Transplant business logic (fct_*.R and utils_*.R) into an external package (tentatively '{inkit}', see https://github.com/The-Strategy-Unit/nhp_planning/issues/417) and import them back here.

What about functions that will be in tpma-explorer but not nhp_inputs (e.g. LA-specific requests), do they belong there too?

Ideally we only want application logic in this app.