fix(sdk): export SerdesContext type in index.ts (#423)

*Issue #, if available:*

N/A

*Description of changes:*

When implementing the `Serdes` interface, it was annoying to declare the
type of the `context` parameter on the `serialize` and `deserialize`
functions without an easily importable reference to `SerdesContext`.

This change addresses that by just adding `SerdesContext` to the exports
in the `packages/aws-durable-execution-sdk-js/src/index.ts` file.


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choice.

Author: bilalq

packages/aws-durable-execution-sdk-js/src/index.ts

@@ -50,6 +50,7 @@ export {
   createClassSerdes,
   createClassSerdesWithDates,
   Serdes,
+  SerdesContext,
 } from "./utils/serdes/serdes";
 export { DurableExecutionApiClient } from "./durable-execution-api-client/durable-execution-api-client";
 export {