Improve vendored FlatBuffers

[oberstet]

Feature Description

Add a version() function to autobahn.flatbuffers that returns the exact git version of the vendored FlatBuffers runtime as a structured tuple. This enables downstream packages (like zlmdb) to verify version compatibility at runtime.

Motivation

The WAMP ecosystem packages (autobahn-python, zlmdb, cfxdb, crossbar) all vendor the FlatBuffers Python runtime from the same upstream source (google/flatbuffers). Currently there's no programmatic way to check which version is vendored, making it difficult to:

1. Verify version compatibility between packages
2. Debug serialization issues that may arise from version mismatches
3. Implement runtime version checks (e.g., zlmdb.check_autobahn_flatbuffers_version_in_sync())

Checklist

• I have searched existing issues to avoid duplicates
• I have described the problem clearly
• I have provided use cases
• I have considered alternatives
• I have assessed impact and breaking changes

[oberstet]

implemented via #1822

autobahn.check_zlmdb_flatbuffers_version_in_sync()

https://github.com/crossbario/autobahn-python/blob/5e4f7e086a074892231377f465099946e8f66ac4/src/autobahn/__init__.py#L38C5-L38C44