PARQUET-604: Add writer headers to installation

[xhochy]

No description provided.

[wesm]

+1, thanks

[wesm]

We're also going to want to create a public API for file writing https://github.com/apache/parquet-cpp/tree/master/src/parquet/api

I'm sort of lukewarm on the api directory; I would be fine with a parquet/parquet.h (or parquet/api.h) with everything public in it. let me know what you think

[xhochy]

I'm okish with the api directory but would be -1 on the single parquet/parquet.h header. We already have some name collisions between Arrow and Parquet and although we have them nicely split up into namespaces, I would prefer to import only the really needed declarations.