feat!: migrate schema to core

[Gozala]

Overview

• Migrates schema from validator to the core
  • I needed it in the transport layer to deal with workflow versions etc.. and pulling in validator there made no sense.
  • Creating separate PR only with this to reduce amount of changes bundled with transport logic
• Adds Variant schema for working with Variant types

[gobengo]

Is there a reason we have to have the _ special case, instead of specifying this default behavior as a whole other parameter not inside VariantChoices?
One downside is it seems to preclude using _ as a variant name (not a huge loss, but seems worth avoiding if possible). Is there an upside to specifying the default behavior via a reserved key name?

[Gozala]

It is very common in typed languages with pattern matching to use _ as a "default" branch e.g. rust does this, elm does this haskell does this, even ucan invocation spec does it in some sense, so it seemed reasonable to do it here.

One downside is it seems to preclude using _ as a variant name (not a huge loss, but seems worth avoiding if possible).

I think using _ as branch name is bad enough idea that I don't actually feel it's a real loss.

specifying this default behavior as a whole other parameter not inside VariantChoices

It is possible and I have tried that in fact, but it made inference types and variant function a lot more complicated and brittle because you ended up with this unnamed branch. In the end it felt clunky so I've chose to go with _ instead.

[gobengo]

but it made inference types and variant function a lot more complicated and brittle

I was guessing maybe this was why (but wanted to confirm), and +1 for avoiding the brittleness.
I didn't realize that _ is common in other languages. Thanks for explaining that.