[crashtracker] rationalize config/metadata structs in API

[danielsn]

          So, I don't think this needs to be addressed by this PR (or ever), but it's somewhat weird to me this "config"/"receiver_config"/"metadata" split right now.

All three are always required to be provided by the caller, so it's kinda weird that the caller needs to know about three different types, which end up just being different kinds of configuration.

Originally posted by @ivoanjo in #551 (comment)