feat: add PEP 561 py.typed marker to support mypy type analysis (#563)

[ShyamD2]

Issue #, if available:
Fixes #563
Description of changes:
Adds PEP 561 compliance so downstream projects using mypy can analyze aws_encryption_sdk without receiving error: Skipping analyzing "aws_encryption_sdk": module is installed, but missing library stubs or py.typed marker.

Summary of Changes:

• Added src/aws_encryption_sdk/py.typed containing the partial marker per PEP 561.
• Updated setup.py to include package_data={"aws_encryption_sdk": ["py.typed"]} so wheel packages bundle the marker.
• Updated MANIFEST.in with include src/aws_encryption_sdk/py.typed for source distribution packaging.

Verification:

Tested with mypy analyzing public client initialization:

$ mypy -c "import aws_encryption_sdk; client = aws_encryption_sdk.EncryptionSDKClient()"
Success: no issues found in 1 source file