feat(rust/sedona-raster-gdal): add rs_asraster

[Kontinuation]

Summary

• add the GDAL-backed rs_asraster raster function
• normalize the SQL name to rs_asraster
• add SQL reference docs and a dedicated Criterion benchmark

Dependency

• Independent branch based on main

Testing

• cargo clippy -p sedona-raster-gdal -p sedona --all-targets -- -D warnings
• cargo bench -p sedona-raster-gdal --bench rs_as_raster --no-run
• cargo test -p sedona-raster-gdal
  • blocked in this environment by existing missing raster fixture files in other tests
• PATH="/home/kontinuation/workspace/github/apache/sedona-db/.venv/bin:$PATH" quarto render docs/reference/sql/rs_asraster.qmd
  • front matter validated; example execution is currently blocked by an existing sedonadb._lib Python import mismatch in this environment