Design a general extension mechanism for Parquet

[asfimport]

Design a general extension mechanism for Parquet with the following requirements:

• old readers must be able to parse files written with the extension

• new readers must be able to parse files written with/without the extension and deterministically detect if the extension is present or not

  Initial ideas: https://docs.google.com/document/d/1PQpY418LkIDHMFYCY8ne_G-CFpThK15LLpzWYbc7rFU/edit

Reporter: Alkis Evlogimenos / @alkis

PRs and other links:

• GitHub Pull Request #254

_{Note: This issue was originally created as PARQUET-2492. Please see the migration documentation for further details.}