flatc support for the FlatBuffer submodule

(the following is a slightly rewritten version of a slack post at the arrow-dev channel)

I am interested in contributing to getting [flatc](https://google.github.io/flatbuffers/flatbuffers_guide_using_schema_compiler.html) support for julia.

I have a [WIP fork of the official flatbuffers repo](https://github.com/jonalm/flatbuffers) which provides some flatc support for the flatbuffer implementation in Arrow.jl. I’ve hijacket the go implementation (I haven’t figured out how to add a new cli option to the flatc command yet). The example of the features which are currently working are shown in this [gist](https://gist.github.com/jonalm/aa2ee7a0346558827218f012c0544272) (i.e. some serialize-deserialize tests). And the code output is shown [here](https://gist.github.com/jonalm/1315ed53f0205ea6e2d97531c78d6b26) . The functionality is basically inspired by the files at `/src/metadata`, and are not complete wrt flatbuffer features, as I’m a bit uncertain about how to proceed with, e.g. union types, mutation functionality, getting struct fields from structs etc.

Before I continue this work, I would like to know whether this work would be supported by the main devs of Arrow.jl, and also if would be possible to move the FlatBuffer submodule of Arrow.jl into it's own repo?

Start with the FlatBuffers schema compiler documentation and the existing Arrow.jl implementation under /src/metadata, then review the linked fork and serialize-deserialize examples. The issue does not define a settled scope; completion would require agreeing on flatc integration, supported features such as unions and mutation, and the submodule's repository ownership.

Written by the indexing model from the issue text.