This commit adds support for the Fetch API's referrer and integrity options when using HttpClient with the withFetch provider.

The change includes:

• Added referrer and integrity to the HttpRequestInit interface
• Updated FetchBackend to forward these options to the native fetch call
• Added unit tests to ensure the options are correctly passed and respected

Motivation / Use Cases

The referrer and integrity options enhance request control and security:

• referrer: Customize or restrict the Referer header sent with a request ('no-referrer', 'origin', or a specific URL)
• integrity: Enables Subresource Integrity (SRI) by verifying fetched content against a cryptographic hash, ensuring resource integrity and preventing tampering

Use cases include:

• Ensuring sensitive requests do not leak origin data (referrer: 'no-referrer')
• Applying SRI checks for assets like scripts or styles (integrity: 'sha512-...')
• Tightening security on resource fetching in production environments

Proposed Solution

• Add referrer and integrity to the HttpRequestInit interface
• Update the HttpRequest and FetchBackend to support and pass these options
• Retain backward compatibility with existing HttpClient behavior

Examples of New Usage

// Request without referrer header and with integrity check
http.get('/scripts/app.js', {
  referrer: 'no-referrer',
  integrity: 'sha512-...'
}).subscribe();