Type-safe variant payloads

[Tymek]

Describe the feature request

I'd like Unleash to have the ability to conform the variant payload (JSON) to a schema.

Background

With new variants in strategies I'd like to have client SDKs that can be type safe.

https://unleash-community.slack.com/archives/C03GZBSJV9A/p1689704431836109

Solution suggestions

A way to get schema in a format, that can be used across multiple programming languages, to generate types. We already have experimental CLI in Next.js SDK, but it doesn't work with variants in strategies yet.

[sighphyre]

@Tymek What do you want to do with this one? Currently it's clogging up our TODO backlog, can we mark this as "maybe later" so the bot doesn't close it?