This continues work start in open-telemetry#324 for Client.

The interface has the following downsides:
- Impossible to define non-trivial default behavior. Here is an example where it was needed: open-telemetry#269 (comment)
- Adding new callbacks requires expanding the interface, which is a breaking change for existing client users.

Getting rid of the interface and keeping just a struct
for callbacks solves both problems:
- Arbitrarily complex default behavior can be now defined on the struct if the user does not provide the particular callback func.
- Adding new callback funcs is not a braking change, existing users won't be affected.