config: package settings in dedicated settings.go files

[bt-justtrack]

To improve searchability / introspection into where our settings come from for different services we want to move these settings structs into a dedicated settings.go file for each package. Optionally if there are too many settings we want a settings_*.go file structure.

[bt-justtrack]

Note: pkg/kvstore/settings.go features two InMemory(Configuration|Settings) having the same fields but the one used in the constructor does not have defaults set.