joinp: refactor --cache=schema option

[jqnatividad]

    --cache-schema <arg>   Create and cache Polars schema JSON files.
                           ‎ -2: treat all columns as String. A Polars schema file is created & cached.
                           ‎ -1: treat all columns as String. No Polars schema file is created.
                             0: do not cache Polars schema.
                             1: cache Polars schema
                                If set to 1 and the schema file/s do not exist, it will check if a
                                stats cache is available. If so, it will use it to derive a Polars
                                schema and save it. If there's no stats cache, it will infer the
                                schema using --infer-len and save the inferred schemas. 
                                Each schema file will have the same file stem as the corresponding
                                input file, with the extension ".pschema.json"
                                (data.csv's Polars schema file will be data.pschema.json)
                                If the file/s exists, it will load the schema instead of inferring it
                                (ignoring --infer-len) and attempt to use it for each corresponding
                                Polars "table" with the same file stem.

closes #2369