Make sure current API could be later extended to support charsets other than UTF-8

[fzhinkin]

We're not planning to support charsets in the near future (and if we are, then the Stdlib is a better place for them). However, the current API should allow supporting them later without breaking the existing API.

A possible API shape for Charset-API needs to be defined; with that definition in mind, we need to check if Charset could be organically integrated into kotlinx-io in the future.