Make TransformConfig into a trait

[rukai]

To achieve this, we sprinkle in some dtolnay magic crates:

• typetag - we will always need this to magically hook up our traits to serde
• async_trait - the rust team seem pretty intent on landing some form of async traits one day so we should eventually be able to remove this

And finally we need a custom deserialize implementation for our chain config.
The problem was that typetagged structs with no fields would not deserialize from a yaml string.
e.g.
this would fail to deserialize:

chain_config:
  redis_chain:
    - NullSink

but this would work fine:

chain_config:
  redis_chain:
    - NullSink: {}

I do not really understand what the vec_transform_config custom deserializer is doing, but it clearly does fix the issue, allowing us to write our config as NullSink instead of NullSink: {}.
I can do a thorough investigation to better understand it if needed.
The implementation was a suggestion from dtolnay: dtolnay/typetag#40 (comment)
This solution does add a bunch of complexity but it is at least hidden away in its own module and doesn't affect users of the API.