Add user-facing CheckpointMetadata classes and Internal classes for IO.

PiperOrigin-RevId: 691962281

checkpoint/CHANGELOG.md

@@ -7,11 +7,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Add `RootMetadata` and `StepMetadata` classes as ways for the user to
+interface with checkpoint metadata at various levels.
+- Add `root_metadata_serialization`, and `step_metadata_io` modules that contain utilities
+to perform de/serialization for `RootMetadata` and `StepMetadata`.
+
 ### Changed
 - Create `Composite` class, which `CompositeArgs` now subclasses.
 - Move `type_handlers` to `_src/serialization`
 - Add notes to Barrier error `XlaRuntimeError(DEADLINE_EXCEEDED)` with
  actionable info.
+- Rename `CheckpointMetadataStore` to `MetadataStore`, and change methods to
+accept and return metadata as dictionaries.
 
 ## [0.8.0] - 2024-10-29