ARROW-7085: Custom column builder for CSV reader

[fexolm]

As discussed in ARROW-7085, I've implemented one of the ways to provide custom column builder to a column.
However, it doesn't look like final solution for me. Hope to hear your thought about possible implementation.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-7085

[fexolm]

@pitrou @wesm PTAL

[pitrou]

@fexolm The Parser, ColumnBuilder.. APIs are currently internal, and slightly messy. We'll have to think about cleaning them up before making them public.

[fexolm]

@pitrou the other option is to define a class inherited from ExtentionType with an additional method for creating column builder for that type.

This solution wouldn't require changing stable API.
I can make a sample implementation if it sounds better for you.

[pitrou]

No, this would have the same problem since ColumnBuilder takes a BlockParser.

[anton-malakhov]

@pitrou We would not mind if this ExtensionType based approach is implemented in internal namespace as well, so it will not appear as a stable API right away.
[updated] I mean ExtensionType is already defined. What we need is additional flexibility in of its usage which will not touch public API but will allow to define our custom converter even if it is internal or uses internal inputs at the moment,

[wesm]

Closing this PR since it has grown stale. I think it would be worth defining a public API for this, but we're going to have to take time to propose clean, well-documented interfaces rather than poking through internal APIs into the public API without any changes