dagql: generalize and unify around Result (formerly known as Instance)

[sipsma]

Another big carve-out from persistent cache work, this change:

1. Generalizes (and renames) dagql.Instance to be able to wrap any type (whereas previously it could only wrap objects)
   • Instance is renamed to -> Result, which is a bit more clear IMO especially once it is used in the persistent cache PR for also wrapping "cache results"
   • Additionally, there are now two types: dagql.Result and dagql.ObjectResult (which is a dagql.Result plus some more object-specific methods)
   • I wanted to make these interfaces, but Go reflection that we use for figuring out API types is just not really usable with interfaces that have a type param, despite many attempts to find a path.
2. Does some refactoring so that internally all field implementations return an Result, and various impacted parts of the codebase

The motivation is that the persistent cache was currently introducing even more wrapper types to support carrying around cache-related metadata (i.e. ID in db, etc.), which was rapidly devolving into insanity mixed in with all our existing types, wrappers, etc.

I decided to centralize everything there around the existing Instance types, which makes sense since Instance is already a wrapper for ID-metadata, which is very related to the cache metadata.

However, doing that required very substantial updates, most important support for wrapping any type, not just objects.

• i.e. if a field returns a String, we still want to be able to cache it and reference it, which in this new world means it needs to be wrapped as an Instance.

@jedevc mentioned that he had wanted Instance to be able to wrap any type for telemetry-related purposes too, so hopefully this should support that too.

[jedevc]

Maybe the PR should be renamed?