feat: Add SnapshotSummaries (#1085)

## Which issue does this PR close?

- Related to #724 

## What changes are included in this PR?
This is the building block to implementing snapshot summaries. Most of
the implementation code is built off of @barronw 's very nice commit.
Most of the added lines are also just tests. A follow up pr will be
included integrating it with the rest of the codebase.

## Are these changes tested?
Yes, unit test

Author: jonathanc-n

crates/iceberg/src/spec/mod.rs

@@ -24,6 +24,7 @@ mod name_mapping;
 mod partition;
 mod schema;
 mod snapshot;
+mod snapshot_summary;
 mod sort;
 mod statistic_file;
 mod table_metadata;

crates/iceberg/src/spec/partition.rs

@@ -20,11 +20,13 @@
  */
 use std::sync::Arc;
 
+use itertools::Itertools;
 use serde::{Deserialize, Serialize};
 use typed_builder::TypedBuilder;
 
 use super::transform::Transform;
 use super::{NestedField, Schema, SchemaRef, StructType};
+use crate::spec::Struct;
 use crate::{Error, ErrorKind, Result};
 
 pub(crate) const UNPARTITIONED_LAST_ASSIGNED_ID: i32 = 999;
@@ -152,6 +154,30 @@ impl PartitionSpec {
 
         true
     }
+
+    pub(crate) fn partition_to_path(&self, data: &Struct, schema: SchemaRef) -> String {
+        let partition_type = self.partition_type(&schema).unwrap();
+        let field_types = partition_type.fields();
+
+        self.fields
+            .iter()
+            .enumerate()
+            .map(|(i, field)| {
+                let value = if data.is_null_at_index(i) {
+                    None
+                } else {
+                    Some(&data.fields()[i])
+                };
+                format!(
+                    "{}={}",
+                    field.name,
+                    field
+                        .transform
+                        .to_human_string(&field_types[i].field_type, value)
+                )
+            })
+            .join("/")
+    }
 }
 
 /// Reference to [`UnboundPartitionSpec`].
@@ -660,7 +686,7 @@ impl CorePartitionSpecValidator for UnboundPartitionSpecBuilder {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::spec::{PrimitiveType, Type};
+    use crate::spec::{Literal, PrimitiveType, Type};
 
     #[test]
     fn test_partition_spec() {
@@ -1733,4 +1759,30 @@ mod tests {
         assert_eq!(1002, spec.fields[1].field_id);
         assert!(!spec.has_sequential_ids());
     }
+
+    #[test]
+    fn test_partition_to_path() {
+        let schema = Schema::builder()
+            .with_fields(vec![
+                NestedField::required(1, "id", Type::Primitive(PrimitiveType::Int)).into(),
+                NestedField::required(2, "name", Type::Primitive(PrimitiveType::String)).into(),
+            ])
+            .build()
+            .unwrap();
+
+        let spec = PartitionSpec::builder(schema.clone())
+            .add_partition_field("id", "id", Transform::Identity)
+            .unwrap()
+            .add_partition_field("name", "name", Transform::Identity)
+            .unwrap()
+            .build()
+            .unwrap();
+
+        let data = Struct::from_iter([Some(Literal::int(42)), Some(Literal::string("alice"))]);
+
+        assert_eq!(
+            spec.partition_to_path(&data, schema.into()),
+            "id=42/name=\"alice\""
+        );
+    }
 }