Content Encoding does not support multiple comma separated content types

[mattpolzin]

For the OpenAPIKit module only (OpenAPI 3.1 specification), we need to support multiple content types on an Encoding Object.

I think the way to do this without a breaking change will be to introduce a new contentTypes array here: https://github.com/mattpolzin/OpenAPIKit/blob/main/Sources/OpenAPIKit/Content/ContentEncoding.swift#L17.

We can deprecate the singular contentType property but still populate it when possible for backwards compatibility with earlier v3 releases of OpenAPIKit.