build: limit sdist contents to source and metadata files (#789)

## Summary

Mirrors apify/crawlee-python#1890.

The latest crawlee-python beta release failed when uploading the sdist:

```
400 Bad Request — Project size too large. Limit for project 'crawlee' total size is 10 GB.
```

`pyproject.toml` only configured the wheel target, so hatchling's
default sdist bundled the entire repo. Each released sdist was much
larger than the actual source. Combined with the fact that a beta
release is published on every src-touching commit to master, the
cumulative storage quota on PyPI hit the cap fast.

This PR applies the same fix here: an explicit
`[tool.hatch.build.targets.sdist]` that ships only `src/apify_client`
and standard metadata files (`CHANGELOG.md`, `CONTRIBUTING.md`,
`LICENSE`, `README.md`, `pyproject.toml`). Verified locally: built sdist
is ~117 KB and contains only those files.

Tests are intentionally excluded — they need dev-only deps that aren't
installable from a plain sdist anyway.

## Note

PyPI's cap is cumulative across all uploaded files, so the eventual
mitigation requires a [project size limit
increase](https://docs.pypi.org/project-management/storage-limits#requesting-a-project-size-limit-increase)
once a project hits its quota. This PR just keeps future releases from
chewing through quota.