add checks to block main graph create/drop

[aheev]

Closes #541

[aheev]

curious, why were these checks added to drop operator class?

    auto dbManager = main::DatabaseManager::Get(*context);
    auto memoryManager = storage::MemoryManager::Get(*context);

    if (!dbManager->hasGraph(dropInfo.name)) {
        auto message = std::format("Graph {} does not exist.", dropInfo.name);
        switch (dropInfo.conflictAction) {
        case ConflictAction::ON_CONFLICT_DO_NOTHING: {
            appendMessage(message, memoryManager);
            return;
        }
        case ConflictAction::ON_CONFLICT_THROW: {
            throw BinderException(message);
        }
        default:
            UNREACHABLE_CODE;
        }
    }

    if (dbManager->hasDefaultGraph() && StringUtils::getUpper(dbManager->getDefaultGraphName()) ==
                                            StringUtils::getUpper(dropInfo.name)) {
        dbManager->clearDefaultGraph();
    }

    dbManager->dropGraph(dropInfo.name, const_cast<main::ClientContext*>(context));
    appendMessage(std::format("Graph {} has been dropped.", dropInfo.name), memoryManager);

it just calls dbManager functions, so, why not move this to dbManager and just call dbManager->dropGraph here?

[adsharma]

Only the defaultGraph checking could be moved to the dbManager. The rest of it should stay in the drop operator because it's user facing cypher behavior.