Add serializer-driven Python measurement API (measurement2) and measurement samples

[KerstinKeller]

Motivation

• Reimplement the Python measurement interface on top of the low-level hdf5 wrapper to support serializer-driven reading and writing across different encodings.
• Provide clear examples for binary, protobuf and string measurement roundtrips to demonstrate the new API and guide migration from protobuf-only logic.

Description

• Introduce a new binary-first measurement API in ecal.measurement2 via src/ecalhdf5/ecal/measurement2/measurement.py implementing MeasurementReader, MeasurementWriter, BinaryChannelReader, BinaryChannelWriter, BinaryEntry, ChannelMetadata and DataTypeInformation.
• Add high-level serializer adapters in ecal.msg: ecal/msg/common/measurement.py, ecal/msg/proto/measurement.py, and ecal/msg/string/measurement.py with corresponding __init__ exports to wire serializer Serializer/Deserializer implementations to the binary core.
• Add measurement sample scripts and README under lang/python/samples/measurement/ (binary_write.py, binary_read.py, person_write.py, person_read.py, string_write.py, string_read.py and README.md) demonstrating usage for binary, protobuf and string channels.
• Update CMakeLists.txt entries to include the new source files and add an IMPLEMENTATION_PLAN.md describing the design and rollout plan.

Testing

• No automated tests were executed as part of this change.

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Codex Task