Add DataAdministrator role and optional contributor fields to support EMBER data management use cases

The `Contributor` model and `RoleType` enum in `dandischema/models.py` already cover most contributor types like `Author`, `DataCurator`, and `DataCollector`. For EMBER-DANDI, there are also contributors who help with data ingestion, embargo coordination, and metadata validation work that doesn’t always fit cleanly into the existing roles.

To better reflect these contributions, I’m proposing a few updates to the contributor roles and related metadata fields, based on notes and comments in the EMBERSet Draft Metadata Google Sheet (https://docs.google.com/spreadsheets/d/1eQTHz6WSTsT84_FXOs2g53ySDEmSy8LtvFN_bic8xB8/edit?gid=0#gid=0) in the “contributor” (now “role”) row.