[Feature]: Add composable SerDes transformation pipelines

## What would you like?

Add a composable SerDes processing pipeline while preserving the existing object-to-string `SerDes[T]` API as the value-codec boundary.

Today, transformations such as filesystem offloading, compression, encryption, signing, or custom envelopes must each wrap and own another complete `SerDes`. This makes independent transformations difficult to reuse or reorder.

The proposed model is:

```text
value -> root SerDes -> string stage -> string stage -> checkpoint
value <- root SerDes <- string stage <- string stage <- checkpoint
```

## Possible Implementation

- Introduce a synchronous `SerDesStage` protocol whose serialize/deserialize methods accept and return `str` plus `SerDesContext`.
- Add a `ComposableSerDes` or composition helper that starts with one existing `SerDes[T]`, applies stages in forward order during serialization, and reverses them during deserialization.
- Require stages to emit a self-identifying/versioned format. During deserialization, a stage should:
- reverse valid input in its own format;
- raise `SerDesError` for malformed input that identifies itself as that stage;
- return unrecognized input unchanged.
- Keep existing `SerDes` implementations and configuration APIs backward compatible.
- Allow `FileSystemSerDes` functionality to be exposed as a stage so callers can choose any root value codec and can place compression, encryption, or signing before or after storage as appropriate.
- Add tests for stage order, pass-through of external invocation input, null/empty values, malformed recognized envelopes, and custom value types.

## Is this a breaking change?

No.

## Does this require an RFC?

Yes.

## Additional Context

Python's current `FileSystemSerDes` already supports a configurable inner value SerDes. A pipeline would separate that value-codec responsibility from reusable string transformations instead of adding another nested-wrapper layer. A comparable Java design is being developed in aws/aws-durable-execution-sdk-java#648.

Start by reading the existing SerDes and FileSystemSerDes implementations and their configuration APIs to understand the current value-codec boundary. Define the stage protocol and composition semantics through the required RFC, then add tests covering ordering, pass-through input, null and empty values, malformed recognized envelopes, and custom value types while preserving backward compatibility.

Written by the indexing model from the issue text.