feat: implement multiple keys for segment distribution

src/ir.rs

src/ir/distribution.rs

src/ir/distribution/tests.rs

src/ir/helpers.rs

src/ir/operator/tests.rs

src/ir/relation.rs

src/ir/relation/tests.rs

tests/artifactory/ir/expression/shrink_dist_key_1.yaml

tests/artifactory/ir/expression/shrink_dist_key_2.yaml

tests/artifactory/ir/expression/shuffle_dist_key.yaml

...s/artifactory/ir/expression/union_fallback_to_random.yaml

tests/artifactory/ir/expression/union_preserve_dist.yaml

tests/artifactory/ir/operator/output_aliases.yaml

tests/artifactory/ir/operator/output_aliases_duplicates.yaml

tests/artifactory/ir/operator/output_aliases_oor.yaml

...ifactory/ir/operator/output_aliases_unsupported_type.yaml

tests/artifactory/ir/operator/projection.yaml

tests/artifactory/ir/operator/scan_rel.yaml

tests/artifactory/ir/operator/selection.yaml

tests/artifactory/ir/operator/sub_query.yaml