Support materializing clr enums from strings

[natemcmaster]

Similar to #3620 but a slightly different scenario. Possible overlap with #242

Example usage scenario: SqlServerDatabaseModelFactory.ConvertToReferentialAction()
Would be nice to define simple mapping for enum values from well-known database strings.

                case "RESTRICT":
                    return ReferentialAction.Restrict;

                case "CASCADE":
                    return ReferentialAction.Cascade;

                case "SET_NULL":
                    return ReferentialAction.SetNull;

                case "SET_DEFAULT":
                    return ReferentialAction.SetDefault;

                case "NO_ACTION":
                    return ReferentialAction.NoAction;

[rowanmiller]

Triage: Agreed that this is covered by #242 since it is not a direct matching of column value to enum value name.