Store graphQL query field names in Entity object to avoid duplicate determination

DAB startup validates that no two entities define GraphQL.Singular/GraphQL.Plural with overlapping values. e.g. GraphQL Query Fields (`entity_by_pk` and `entities`) are not duplicated.

A separate startup step, GraphQL schema generation, reevaluates the query field names.

The above two operations perform redundant operations. DAB should remember the query field names the first time they are resolved and re-use those values in any subsequent validations/startup operations to avoid re-calculation.