feat: add the design implementation for the sd artifacts download [skip ci] (#3220)

Author: VonnyJap

design/sd-download-artifacts.md

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+# SD Download Zipped Artifacts
+
+#### Context
+
+The current [API endpoint](https://github.com/screwdriver-cd/screwdriver/blob/master/plugins/builds/artifacts/get.js) allows downloading artifacts for a single file. This is quite limited, as users often need to download multiple files simultaneously.
+
+#### Proposal
+
+###### Asynchronous Implementation (preferred for a foolproof solution)
+
+```mermaid
+sequenceDiagram
+  Client->>API: Download dir
+  API->>DB: Save request
+  API->>Client: Return 201 with a link to get download status
+  Zipper->>DB: Consume request
+  create participant Store
+  Zipper->>Store: Stream files
+  destroy Store
+  Zipper->>Store: Close connection
+  Zipper->>Zipper: zipping
+  create participant S3
+  Zipper->>S3: upload zipped file
+  destroy S3
+  S3->>Zipper: send the URL info
+  destroy Zipper
+  Zipper->>DB: update request with S3 URL
+  Client->>API: Download dir status
+  destroy DB
+  API->>DB: Get download URL based on request ID
+  API->>Client: Return 200 with S3 URL
+```
+
+###### Async Implementation Flows
+
+1. User requests to download a directory.
+2. The client sends a download directory request to the API.
+3. The API saves the request in the database.
+4. The API returns a 201 response to the client with a link to check the download status.
+5. The Zipper service consumes the request from the database.
+6. The Zipper service calls Store and streams the files from the store.
+7. The Zipper service starts zipping the streamed files.
+8. The Zipper service uploads the zipped file to S3.
+9. The S3 sends the URL information back to the Zipper service.
+10. The Zipper service updates the request in the database with the S3 URL.
+11. The client sends a request to check the download directory status to the API.
+12. The API retrieves the download URL based on the request ID from the database.
+13. The API returns a 200 response to the client with the S3 URL.
+
+###### Synchronous Implementation (fast development time)
+
+```mermaid
+sequenceDiagram
+  Client->>API: Download dir
+  loop Directory tree
+    API->>Store: Stream files
+  end
+  API->>API: zipping
+  API->>Client: return the zipped file
+```
+
+###### Synchronous Implementation Flows
+
+1. User requests to download a directory.
+2. The client sends a download directory request to the API.
+3. The API begins streaming the files from the store in a loop, recursively traversing the entire directory tree.
+4. The API zips the streamed files.
+5. The API returns the zipped file to the client.
+6. The client receives the zipped file.
+
+#### Design Caveat
+
+Depending on the number of files being streamed, there’s a potential risk of running into memory issues. To prevent this, we should implement a check to estimate the total download size, such as 1GB.