feat(gorm): implement Translate for Spanner gRPC errors

[Gage-McGuire]

Implementation of #243

Only added the mapping of AlreadyExists to gorm.ErrDuplicatedKey. There is a way to also add mappings for other error types but requires evaluating the desc of the returned errors from spanner. Which IMO could become fragile since Google states desc could be changed at anytime... (example below)

switch code {
    case codes.FailedPrecondition:
        if strings.Contains(msg, "Foreign key constraint") {
            return gorm.ErrForeignKeyViolated
        }
    case codes.OutOfRange:
        if strings.Contains(msg, "Check constraint") {
            return gorm.ErrCheckConstraintViolated
        }
    case codes.InvalidArgument:
        if strings.Contains(msg, "Unrecognized name") {
            return gorm.ErrInvalidField
        }
    }