Make it possible to exclude pyarrow dep (#276)

* Move pyarrow to an in method import.

This allows client to exclude the pyarrow dep if they don't need it. Saved ~80MB and more compatible with older systems.

Will still get a runtime error if they exclude it, then try to use it.

Still works as expected unless users go out of their way to manually exclude this dependency (I'm not removing the dep, you need to manually exclude it).

* Make pyarrow optional, and show an error if not installed when trying to use parquet files.

Example Error
```
$ uv run python test_pyarrow.py
Expected ImportError: pyarrow is not installed and is required to use parquet files. Please install it via `pip install together[pyarrow]`
```

Confirmed installing resolves issue:
```
uv pip install "dist/together-1.5.0-py3-none-any.whl[pyarrow]"
Resolved 33 packages in 394ms
Installed 1 package in 30ms
 + pyarrow==20.0.0
```

* fix formatting issue

* Ran `poetry lock`

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Co-authored-by: orangetin <[email protected]>

Author: scosman