feat: Add support for Float16 dtype

[dsprenkels]

This PR implements support for 16-bit floating point types. It creates a pf16 (i.e., "polars f16") newtype (to prevent confusion with other f16 definitions). It is based on the f16 type from the half crate, but it is newtype'd such that we can impl an vast amount of traits that we need.

Still TODO

• Check if all [amber] Check if this code is covered spots are covered by tests
• Read through all the code once more
• Check the coverage report to see if there are spots that I do not trust well enough
• Benchmark the speed and memory usage of some basic operations
• Update the reference API
• Update the user guide
• Add some interop tests
  • Polars IPC <-> pyarrow feather
  • Polars parquet <-> pyarrow parquet
  • Polars arrow <-> pyarrow
  • Polars CSV <-> ?? (maybe just roundtrip)
  • Parquet roundtrip
  • IPC roundtrip
  • JSON roundtrip

Bikeshedding suggestions

• Is polars-utils the right place to host pf16?

Fixes: #7288
Related: #21803 (not fixed)

[ritchie46]

Polars CSV <-> ?? (maybe just roundtrip)

We have post-read casts, I think that would be fine. This is done when a user passes a f16 dtype as schema. Round tripping without passing a schema is impossible as we don't encode dtypes.