[C#] Update Flat Buffers code to latest version #21091
Description
In order to support zero-copy reads, we should update to the latest Google Flat Buffers code. A recent change now allows C# support for directly reading and writing to memory other than byte[] which will make reading native memory using Memory<byte> possible.
Along with this update, we should mark the flat buffers types as internal, since they are an implementation detail of the library. From an API perspective, it is confusing to see multiple public types named "Schema", "Field", "RecordBatch" etc.
Reporter: Eric Erhardt / @eerhardt
Assignee: Eric Erhardt / @eerhardt
PRs and other links:
Note: This issue was originally created as ARROW-4543. Please see the migration documentation for further details.