feat(map): Implement Map Operation

[ayushiahjolia]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Issue Link, if available

#39

Wire up the map() / mapAsync() API in DurableContext to a working MapOperation that executes user functions concurrently across collection items using child contexts.

Description

• MapOperation: new class extending BaseConcurrentOperation that creates a ChildContextOperation branch per item via branchInternal(), with get() directly waiting on each branch and aggregating into BatchResult
• MapConfig: add optional serDes field with fallback to handler default in DurableContext.mapAsync()
• DurableContext: replace mapAsync() TODO stub with MapOperation wiring
• OperationSubType: add MAP_ITERATION enum value for branch subtype
• ChildContextOperation: consolidate MAP/MAP_ITERATION/PARALLEL/RUN_IN_CHILD_CONTEXT failure cases in get() switch
• BaseConcurrentOperation: use sendOperationUpdateAsync in checkpointResult
• MapIntegrationTest: two integration tests using LocalDurableTestRunner verifying simple map and map with durable steps inside branches

Next changes,

• Parent checkpointing
• maxConcurrency
• CompletionConfig

Demo/Screenshots

Checklist

• I have filled out every section of the PR template
• I have thoroughly tested this change

Testing

Unit Tests

Have unit tests been written for these changes? N/A

Integration Tests

Have integration tests been written for these changes?
Added a basic test.

Examples

Has a new example been added for the change? (if applicable) N/A

[zhongkechen]

Good job. First working version of map