python(feat): add MCAP imports

[wei-qlu]

Summary

Adds MCAP (.mcap) import support to sift_client for ROS 2 topics (ros2msg/cdr)

job = client.data_import.import_from_path("recording.mcap", asset=my_asset)

detect_config reads the file's channels locally without decoding messages, giving one entry per flattened field named <topic>.<field_path>. Edit it to select, rename, or retype channels; leave data empty to import everything.

Variable-cardinality fields are typed BYTES, with complex_types_import_mode deciding what each becomes, matching Parquet. Undecodable topics are skipped with a warning unless parse_error_policy is IGNORE_ERROR, which fails the import.

Adds the MCAP config types, DataTypeKey.MCAP, an example, and a new mcap extra.

Testing

Unit tests covering schema flattening, type mapping, topic rules, the three scan paths, and each complex types mode.

Manual imports with MCAP files

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-755/

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