[Reporting] Abstract reports storage (#106821)

* Add duplex content stream
* Add content stream factory
* Move report contents gathering and writing to the content stream
* Update jobs executors to use content stream instead of returning report contents
# Conflicts:
#	x-pack/plugins/reporting/server/export_types/printable_pdf/execute_job/index.test.ts

x-pack/plugins/reporting/common/types.ts

@@ -44,7 +44,7 @@ export interface ReportDocumentHead {
   _primary_term: number;
 }
 
-export interface TaskRunResult {
+export interface ReportOutput {
   content_type: string | null;
   content: string | null;
   size: number;
@@ -53,6 +53,8 @@ export interface TaskRunResult {
   warnings?: string[];
 }
 
+export type TaskRunResult = Omit<ReportOutput, 'content'>;
+
 export interface ReportSource {
   /*
    * Required fields: populated in enqueue_job when the request comes in to
@@ -73,7 +75,7 @@ export interface ReportSource {
   /*
    * `output` is only populated if the report job is completed or failed.
    */
-  output: TaskRunResult | null;
+  output: ReportOutput | null;
 
   /*
    * Optional fields: populated when the job is claimed to execute, and after
@@ -122,20 +124,14 @@ export type JobStatus =
   | 'processing' // Report job has been claimed and is executing
   | 'failed'; // Report was not successful, and all retries are done. Nothing to download.
 
-// payload for retrieving the error message of a failed job
-export interface JobContent {
-  content: TaskRunResult['content'];
-  content_type: false;
-}
-
 /*
  * Info API response: to avoid unnecessary large payloads on a network, the
  * report query results do not include `payload.headers` or `output.content`,
  * which can be long strings of meaningless text
  */
 interface ReportSimple extends Omit<ReportSource, 'payload' | 'output'> {
   payload: Omit<ReportSource['payload'], 'headers'>;
-  output?: Omit<TaskRunResult, 'content'>; // is undefined for report jobs that are not completed
+  output?: Omit<ReportOutput, 'content'>; // is undefined for report jobs that are not completed
 }
 
 /*