Column filter structure

[ackwell]

Sheet endpoints would benefit from a query argument(s) to control the columns that will be returned in the response. While the query arguments are necessarily strings, they should be parsed into some form of structured representation that can be "walked" alongside the schema.

Open questions:

• Schema package has some need for column specifiers on references, is this something relevant to that?
  It likely isn't relevant - reference specifiers are naturally single-column targets, while column filtering must target 0-to-many. Search filter specifiers will probably be more appropriate for this.

[ackwell]

Completed in ac67f40