(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/plugin/ImportSourceAPI.ts

@@ -0,0 +1,53 @@
+/**
+ * API definitions for ImportSource plugins.
+ */
+
+import { GristTable } from './GristTable';
+
+export interface ImportSourceAPI {
+  /**
+   * Returns a promise that resolves to an `ImportSource` which is then passed for import to the
+   * import modal dialog. `undefined` interrupts the workflow and prevent the modal from showing up,
+   * but not an empty list of `ImportSourceItem`. Which is a valid import source and is used in
+   * cases where only options are to be sent to an `ImportProcessAPI` implementation.
+   */
+  getImportSource(): Promise<ImportSource|undefined>;
+}
+
+export interface ImportProcessorAPI {
+  processImport(source: ImportSource): Promise<GristTable[]>;
+}
+
+export interface FileContent {
+  content: any;
+  name: string;
+}
+
+export interface FileListItem {
+  kind: "fileList";
+  // TODO: there're might be a better way to send file content. In particular for electron where
+  // file will then be send from client to server where it shouldn't be really. An idea could be to
+  // expose something similar to `client/lib/upload.ts` to let plugins create upload entries, and
+  // then send only uploads ids over the rpc.
+  files: FileContent[];
+}
+
+export interface URL {
+  kind: "url";
+  url: string;
+}
+
+export interface ImportSource {
+  item: FileListItem | URL;
+
+  /**
+   * The options are only passed within this plugin, nothing else needs to know how they are
+   * serialized. Using JSON.stringify/JSON.parse is a simple approach.
+   */
+  options?: string|Buffer;
+
+  /**
+   * The short description that shows in the import dialog after source have been selected.
+   */
+  description?: string;
+}