feat: add Fastify server for testing image optimization

- Added Fastify as a dependency in package.json.
- Implemented a Fastify server in optimize.e2e-spec.ts to serve a test image.
- Updated test cases to fetch the image from the Fastify server instead of an external URL.
- Adjusted expected Content-Length values for optimized image formats.
- Added test image (znaem.png) to the test-files directory.

Author: LabEG