DB Manager service name

[andreyvelich]

We renamed katib-manager component to katib-db-manager: #1006, but in gRPC API we still name related service Manager: https://github.com/kubeflow/katib/blob/master/pkg/apis/manager/v1beta1/api.proto#L16-L34.

Maybe we should rename this service to DBManager to avoid confusions, since we add only DB-related API there?

This APIs services are under /apis/manager directory which also sounds not clear.

I think we should say that: Katib has gRPC manager which supports:

1. service DBManager - Service to call DB-related APIs.
2. service Suggestion - Service to call Suggestion-related APIs.
3. service EarlyStopping - Service to call Early Stopping-related APIs.

What do you think @gaocegege @johnugeorge ?

/kind question

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