Introduce a process to assure spec changes are implemented by language SIGs

from https://github.com/open-telemetry/oteps/pull/65#issuecomment-600873288:

We should introduce a process to assure all spec (and proto) changes are implemented by all affected language SIGs and don't go unnoticed.