feat(transaction): Implement TransactionAction for FastAppendAction (#1448)

## Which issue does this PR close?

Related Issues:
- #1382 [EPIC]

## What changes are included in this PR?

- Implement `TransactionAction` for `FastAppendAction`
- Changed `SnapshotProduceAction` to `SnapshotProducer`
- The idea is to make `SnapshotProduceAction` an _stateless_ helper
class that helps with fast_append::commit (as discussed
[here](#1400 (comment)))

## Are these changes tested?
Added unit tests

Author: CTTY

crates/iceberg/src/io/object_cache.rs

@@ -266,7 +266,7 @@ mod tests {
             let mut writer = ManifestWriterBuilder::new(
                 self.next_manifest_file(),
                 Some(current_snapshot.snapshot_id()),
-                vec![],
+                None,
                 current_schema.clone(),
                 current_partition_spec.as_ref().clone(),
             )

crates/iceberg/src/scan/mod.rs

@@ -752,7 +752,7 @@ pub mod tests {
             let mut writer = ManifestWriterBuilder::new(
                 self.next_manifest_file(),
                 Some(current_snapshot.snapshot_id()),
-                vec![],
+                None,
                 current_schema.clone(),
                 current_partition_spec.as_ref().clone(),
             )
@@ -964,7 +964,7 @@ pub mod tests {
             let mut writer = ManifestWriterBuilder::new(
                 self.next_manifest_file(),
                 Some(current_snapshot.snapshot_id()),
-                vec![],
+                None,
                 current_schema.clone(),
                 current_partition_spec.as_ref().clone(),
             )

crates/iceberg/src/spec/manifest/mod.rs

@@ -232,7 +232,7 @@ mod tests {
         let mut writer = ManifestWriterBuilder::new(
             output_file,
             Some(1),
-            vec![],
+            None,
             metadata.schema.clone(),
             metadata.partition_spec.clone(),
         )
@@ -417,7 +417,7 @@ mod tests {
         let mut writer = ManifestWriterBuilder::new(
             output_file,
             Some(2),
-            vec![],
+            None,
             metadata.schema.clone(),
             metadata.partition_spec.clone(),
         )
@@ -514,7 +514,7 @@ mod tests {
         let mut writer = ManifestWriterBuilder::new(
             output_file,
             Some(3),
-            vec![],
+            None,
             metadata.schema.clone(),
             metadata.partition_spec.clone(),
         )
@@ -623,7 +623,7 @@ mod tests {
         let mut writer = ManifestWriterBuilder::new(
             output_file,
             Some(2),
-            vec![],
+            None,
             metadata.schema.clone(),
             metadata.partition_spec.clone(),
         )
@@ -731,7 +731,7 @@ mod tests {
         let mut writer = ManifestWriterBuilder::new(
             output_file,
             Some(2),
-            vec![],
+            None,
             metadata.schema.clone(),
             metadata.partition_spec.clone(),
         )
@@ -1010,7 +1010,7 @@ mod tests {
         let mut writer = ManifestWriterBuilder::new(
             output_file,
             Some(1),
-            vec![],
+            None,
             metadata.schema.clone(),
             metadata.partition_spec.clone(),
         )

crates/iceberg/src/spec/manifest/writer.rs

@@ -40,7 +40,7 @@ use crate::{Error, ErrorKind};
 pub struct ManifestWriterBuilder {
     output: OutputFile,
     snapshot_id: Option<i64>,
-    key_metadata: Vec<u8>,
+    key_metadata: Option<Vec<u8>>,
     schema: SchemaRef,
     partition_spec: PartitionSpec,
 }
@@ -50,7 +50,7 @@ impl ManifestWriterBuilder {
     pub fn new(
         output: OutputFile,
         snapshot_id: Option<i64>,
-        key_metadata: Vec<u8>,
+        key_metadata: Option<Vec<u8>>,
         schema: SchemaRef,
         partition_spec: PartitionSpec,
     ) -> Self {
@@ -115,7 +115,7 @@ pub struct ManifestWriter {
 
     min_seq_num: Option<i64>,
 
-    key_metadata: Vec<u8>,
+    key_metadata: Option<Vec<u8>>,
 
     manifest_entries: Vec<ManifestEntry>,
 
@@ -127,7 +127,7 @@ impl ManifestWriter {
     pub(crate) fn new(
         output: OutputFile,
         snapshot_id: Option<i64>,
-        key_metadata: Vec<u8>,
+        key_metadata: Option<Vec<u8>>,
         metadata: ManifestMetadata,
     ) -> Self {
         Self {
@@ -416,7 +416,7 @@ impl ManifestWriter {
             existing_rows_count: Some(self.existing_rows),
             deleted_rows_count: Some(self.deleted_rows),
             partitions: Some(partition_summary),
-            key_metadata: Some(self.key_metadata),
+            key_metadata: self.key_metadata,
         })
     }
 }
@@ -622,7 +622,7 @@ mod tests {
         let mut writer = ManifestWriterBuilder::new(
             output_file,
             Some(3),
-            vec![],
+            None,
             metadata.schema.clone(),
             metadata.partition_spec.clone(),
         )