Fix read_flights() after changes to ANAC data access - #53

Proposed changes:

• Use httr2 to retrieve the ANAC data page.
• Retrieve file URLs directly from ANAC tables instead of constructing them from date and type
• Parse table columns by their headers to accommodate changes in the page structure.

The new function get_flights_files_available() returns a data.table with columns date, type, and url, whereas get_flights_dates_available() is now a simple wrapper around it for backward compatibility ---I suggest removing it from the pipeline entirely.

Furthermore, the result of get_flights_files_available (minus the URL column) could be a useful output for the users, like a metadata/catalog table.

Fixes #52