We extracted implementation of build/bridge metadata attributes to a
separate concern, because in EE `Ci::Bridge` also has metadata
attributes, and we want to build abstraction for storing values in build
metadata table.
This commit segregates interface that is require to make CI/CD entity
processable, like `Ci::Build`. With this change it is not clear and
explicit what methods need to be implement to pass an object to pipeline
processing service.
