[Move `@kbn/config-schema` to server] `stack_connectors`

[afharo]

Summary

Part of https://github.com/elastic/kibana/pull/189476.

We want to avoid @kbn/config-schema from leaking to the browser, and this plugin is using it outside the ./server directory.

I found that the UI only references this code via import type, so I don't expect it to have any impact on bundle metrics, but this PR is still useful to avoid potential future leaks.

For maintainers

• [x] This was checked for breaking API changes and was labeled appropriately