Update Web IDE to receive `extensionMarketplace` settings

MR: feat: Add extensionsGallerySettings to config (!307 - merged)

Description

From Create User-Level Extension Marketplace settings (gitlab#451875 - closed), we should update the BaseConfig to accept extensionMarketplace settings and pass this along to VSCode Fork (that will be consumed through Update VSCode Fork to receive marketplace settings (gitlab-web-ide-vscode-fork#31 - closed)).

Draft:

interface ExtensionMarketplaceConfigEnabled {
  enabled: true;
  // From https://github.com/microsoft/vscode/blob/0e16324eff7e45a43223f9c11dafa1b3768d3174/src/vs/base/common/product.ts#L95
  serviceUrl: string;
  itemUrl: string;
  publisherUrl: string;
  resourceUrlTemplate: string;
  controlUrl: string;
  nlsBaseUrl: string;
}

interface ExtensionMarketplaceConfigDisabled {
  enabled: false;
  message: string; // Is it okay if this contains HTML?? How can we get links into the VSCode fork??
}

type ExtensionMarketplaceConfig = ExtensionMarketplaceConfigEnabled | ExtensionMarketplaceConfigDisabled;