Support DataAPI.jl metadata API

[bkamins]

Hi @quinnj - could you please add to the release plan of Arrow.jl support for https://github.com/JuliaData/DataAPI.jl/pull/48 for the created Arrow tables. Only read methods need to be implemented for Arrow tables:

• DataAPI.metadata
• DataAPI.metadatakeys
• DataAPI.colmetadata
• DataAPI.colmetadatakeys

[bkamins]

@quinnj - we should get back to it as the issue got forgotten (at least by me). In particular we should also add an automatic writing of metadata from objects that support metadata (instead of using metadata and colmetadata keywords in writer).

Do you have any comments on this (or I should just propose a PR)?

[quinnj]

Ah yes, just make a PR; I think it should be pretty straightforward.

[bkamins]

In https://github.com/apache/arrow-julia/pull/481 I have implemented it for Arrow.Table.

My follow-up questions are:

• I was not sure how Arrow.Stream should be handled for metadata reading.
• I was not sure for Arrow.write and Arrow.Writer and Arrow.append how to handle writing metadata. In particular, if one writes to an already existing Arrow file what should be allowed (i.e. if adding/updating metadata should be supported).