Add support for JSON based consumers

[rasviitanen]

I have some use cases where I would like to send JSON in a CDC consumer. Right now, the consumer only accepts Cql, and extracting/processing the CqlRow can be a bit complicated, especially if you want to build some JSON response from it.

I think it would be beneficial if you didn't have to do any conversion client-side in this case. Would it be reasonable to add support for accepting JSON rows instead of Cql in the consumer implementation?

I've implemented a draft for what that could look like: https://github.com/scylladb/scylla-cdc-rust/compare/main...rasviitanen:scylla-cdc-rust:rasviitanen/json-consumer?expand=1.

In short:

1. Add a trait ConsumerType and implement it for Json and Cql
2. Add a marker type to the consumer trait and default it to Cql: Consumer<T = Cql> where T: ConsumerType
3. Change the consume_cdc function to async fn consume_cdc(&mut self, data: T::Value<'_>) -> anyhow::Result<()>. T::Value for Cql will be the current CDCRow and for Json it's basically just a json text string.

If there's any interest from Scylla's side, I'd be happy to submit a PR for it. I'd understand if you wish to implement it some other way, or not have the feature available at all, but this felt decent enough to me!

Thank you!