Add FlatBuffers schema support (.fbs)

[clemensv]

Feature request

Add conversion support for FlatBuffers schema files (.fbs).

Motivation

FlatBuffers is widely used in low-latency systems and mobile/game pipelines; schema conversion would enable smoother integration.

Suggested scope

• Import: bs2a (FlatBuffers -> Avrotize Schema)
• Export: ^G2fbs (Avrotize Schema -> FlatBuffers)
• Handle tables, structs, enums, unions, vectors, namespaces, and scalar mappings
• Capture FlatBuffers-specific metadata where direct mapping is lossy

Acceptance criteria

• CLI commands + docs
• Tests for representative schemas and edge cases
• Clear mapping notes for non-lossless features

Additional requirement: JSON Structure conversions

Please also include JSON Structure conversion support:

• bs2s (FlatBuffers -> JSON Structure)
• s2fbs (JSON Structure -> FlatBuffers)
• Preserve conversion metadata for non-lossless constructs.

[clemensv]

Additional requirement for this issue:
Please include JSON Structure conversion support in scope as well (import to JSON Structure and export from JSON Structure), with tests and documented mapping limitations where needed.