feature: support local sources for processing inputs

[gauravkriplani]

Summary

• support v2-compatible ProcessingInput(source=..., destination=...) construction in SDK v3
• automatically stage local processing inputs through the existing S3 uploader
• preserve service-level s3_input and dataset_definition construction
• keep handwritten compatibility behavior separate from generated AWS service shapes

Problem

SDK v3 requires users to manually construct a nested ProcessingS3Input, even though the processor already knows how to upload non-S3 sources. This restores the convenient local-path workflow without modifying generated shape code.

Testing

• pytest -q sagemaker-core/tests/unit/test_processing.py sagemaker-core/tests/unit/generated/test_shapes.py — 140 passed
• verified local file upload destination and returned S3 URI propagation
• verified both public import paths expose the compatibility constructor
• black --check
• git diff --check

Fixes #5672