Function fetchS3ReadDelegation

fetchS3ReadDelegation(
    keypair: SessionKeypair,
    fileStorageKey: string,
    environment: WelshareApiEnvironment,
): Promise<ReadDelegationResponse>
Fetch a presigned URL for downloading encrypted files from S3 storage.
Parameters
- keypair: SessionKeypair
  The user's session keypair for authentication
- fileStorageKey: string
  The storage key (without welshare:// prefix). Use resolveStorageKey() to convert from URL format.
- environment: WelshareApiEnvironment
  The Welshare API environment configuration
Returns Promise<ReadDelegationResponse>
Promise containing presignedUrl (S3 GET URL)
Example
```
import { WelshareApi, resolveEnvironment } from "@welshare/sdk";

const storageKey = WelshareApi.resolveStorageKey(binaryFile.url);
const { presignedUrl } = await WelshareApi.fetchS3ReadDelegation(
  keypair,
  storageKey,
  resolveEnvironment("production")
);

// Download encrypted file
const response = await fetch(presignedUrl);
const encryptedData = await response.arrayBuffer();
```
Remarks
Side effects:
- Makes HTTP POST to {apiUrl}/auth/delegate/storage?requestType=read
- Creates a self-signed JWT using the keypair for authentication
Security notes:
- Presigned URLs expire after 15 seconds
- Only the file owner (controller_did) or authorized parties can download
- Defined in lib/welshare/fetch-storage-delegation.ts:150