This PR refactors the `OpenRequest` to introduce an `OpenRequestKind`
enum.

It seems most of the fields on `OpenRequest` are mutually-exclusive, so
it is better to model it as an enum rather than using a bunch of
`Option`s.

There are likely more of the existing fields that can be converted into
`OpenRequestKind` variants, but I'm being conservative for this first
pass.

Release Notes:

- N/A