conan integration doesn't use conan2 and doesn't package bazel rules

[peakschris]

I found a few minor issues with the conan integration:

• conanfile.py is a bit out of date, not using the conan2 APIs
• it does not package build_defs.bzl, so bazel clients can't pull flatbuffers from conan and use directly

I have an update, which I'm happy to contribute. I would not be able to integrate this into your automated push to conan central, so it would need some integration work into the github pipeline.

Is this of interest?