support sidx atom

[morphkurt]

This pull request introduces a new SidxBox structure to the mp4box module. The main changes include adding the new module, defining the SidxBox and Segment structures, and implementing necessary traits for reading, writing, and serializing the SidxBox.

New Feature: SidxBox Implementation

• Module Addition:

  • Added pub(crate) mod sidx; to src/mp4box/mod.rs.
  • Added pub use sidx::SidxBox; to src/mp4box/mod.rs.
  • Added SidxBox to the boxtype! macro in src/mp4box/mod.rs.

• SidxBox Structure:

  • Defined the SidxBox structure with fields such as version, flags, reference_id, timescale, earliest_presentation_time, first_offset, reserved, reference_count, and segments.
  • Implemented the Mp4Box, Default, ReadBox, and WriteBox traits for SidxBox.

• Segment Structure:

  • Defined the Segment structure with fields such as reference_type, reference_size, subsegment_duration, starts_with_sap, sap_type, and sap_delta_time.
  • Implemented methods for reading and writing Segment instances.

• Tests:

  • Added tests for SidxBox with both version 0 and version 1 formats to ensure correct serialization and deserialization.