Function fetchS3WriteDelegation

fetchS3WriteDelegation(
    keypair: SessionKeypair,
    metadata: WriteMetadata,
    environment: WelshareApiEnvironment,
): Promise<WriteDelegationResponse>
Fetch a presigned URL for uploading encrypted files to S3 storage.
Parameters
- keypair: SessionKeypair
  The user's session keypair for authentication
- metadata: WriteMetadata
  File metadata including reference, fileName, and fileType
- environment: WelshareApiEnvironment
  The Welshare API environment configuration
Returns Promise<WriteDelegationResponse>
Promise containing presignedUrl (S3 PUT URL) and uploadKey (storage path)
Example
```
import { WelshareApi, resolveEnvironment } from "@welshare/sdk";

const environment = resolveEnvironment("production");
const { presignedUrl, uploadKey } = await WelshareApi.fetchS3WriteDelegation(
  keypair,
  {
    reference: "questionnaire/abc-123/photo",
    fileName: "photo.jpg",
    fileType: "image/jpeg",
  },
  environment
);

// Upload encrypted file to S3
await fetch(presignedUrl, {
  method: "PUT",
  body: encryptedFileData,
  headers: { "Content-Type": "application/octet-stream" },
});
```
Remarks
Side effects:
- Makes HTTP POST to {apiUrl}/auth/delegate/storage?requestType=write
- Creates a self-signed JWT using the keypair for authentication
Security notes:
- Presigned URLs expire after 15 seconds
- The uploadKey format: {network}/user-uploads/{did}/{timestamp}-{fileName}
- Defined in lib/welshare/fetch-storage-delegation.ts:108