[ENG-1985] use share.schema in share.util.graph

[aaxelb]

what

refactor MutableGraph and MutableNode in share.util.graph to get schema info from share.schema (new in #776) instead of from the ShareObject model classes (which will soon be deleted)

deviations

breaking changes in the public interface of MutableGraph and MutableNode:

• remove MutableNode.model, MutableNode.concrete_model
  • instead, use the new MutableNode.schema_type to get type info in the form share.schema.shapes::ShareV2SchemaType
• remove MutableGraph.filter_by_concrete_model
  • instead, use the new MutableGraph.filter_by_concrete_type, which takes a string instead of a model class

jira

ENG-1985