Request: Add support for verbose thread information and configuration

[DeflateAwning]

It would be awesome if you could add the pl.thread_pool_size() method (https://docs.pola.rs/api/python/stable/reference/api/polars.thread_pool_size.html).

It would also be great if you could support setting the POLARS_MAX_THREADS configuration (e.g., via an environment variable). It's not currently in the list of supported environment variables.

[ankane]

Added in the commit above.

[DeflateAwning]

Thank you!