feat(entity_tagger): Support generic tag interface (#2696)
* refactor(entity_tag): generic tag operation
- adds a generic tag method to EntityTagger.
- refactors ElasticSearchEntityTagger to delegate to
tagging AtomicOperationConverters for tagging operation
construction.
* fix(entity_tag): Generic tag method to use namespace over category
Since `category` is specific to only alert/notice entity tags,
I changed the generic tag interface to use namespace instead.
Same thing as Cam's PR, but with above change and comments addressed.
https://github.com/spinnaker/clouddriver/pull/2540
