Use PublishJson from outside the crate

[siemtim-dev]

PublishJson is not constructible from outside the crate because the fields are only pub(crate). I asses a PublishJson::new() function to allow the construction of PublishJson from outside the crate.

[Mossop]

Topic::with_json exists for constructing this, is there a need for the additional method?

[siemtim-dev]

Ah, that's right. It is a bit unintuitive as a Method of Topic. I will close this MR.