Reconsider config object names

[colmsnowplow]

For sources, where we have fully segmented configs into their pkg, we name all the config objects configuration. This made the linter stop complaining about verbose names.

Elsewhere, we have some inconsistent names (eg. KafkaConfig and HTTPTargetConfig for the Kafka and HTTP targets, respectively).

Thinking about clear documentation for the project, we should do something about both of these, and consider making all configuration options have a consistent name which specifies exactly what the config is for. That would play nicely with automating API docs using go doc or similar tools.

I don't mind going against the linter's wishes if it gives us better/easier to maintain documentation.

[colmsnowplow]

Additionally, for sources (and when we get round to doing similar to the others), even though configuration objects don't need to be public, if making them public might provide some value from a documentation perspective, then they should be public.