Cross-compilation to native for `tapir-apispec-docs` and `tapir-openapi-docs`

[j-mie6]

This PR cross-builds for Scala native for these two projects.

As a by-product, a bunch of cross-builds have to be added for either Scala 2 or Scala-native for other projects: this is largely unnecessary, since it's just because of a dependency on tests.

[j-mie6]

Ok, ok... I take it from the error this is a dependency for core? Or is it via test?

[adamw]

This is failing since there are no cats releases for scala native 0.5. This will be available quite soon I guess, so we'll revisit then