Add experimental kotlinx-io core format

[hfhbd]

What is your use-case and why do you need this feature?
The current experimental kotlinx.io integration is only available for Json, and there is no core kotlinx.io SerialFormat.
But there are many 3rd party formats available that would also benefit from the experimental kotlinx.io integration. But each author needs to integrate the support by itself (mostly by copying the json api).
Additionally, if you want to consume multiple formats with io support, there is no unify function because there is no interface, instead you need to handle every format function manually.

Use-case
I want to use multiple serialization formats with Ktor, that already provides the experimental json-io format, but it is currently limited to Json because there is no core artifact with kotlinx-io integration (yet).

Describe the solution you'd like
Add an experimental SerialFormat that is compatible with kotlinx.io.

This SerialFormat should be a new interface in a new experimental artifact and the json-io implementation should implement it.

Because it is experimental, it is absolutely fine that this new SerialFormat won't be stable.