Add http/2 support

[aryamansingh2008]

Motivation / Background

The current subscriber implementation directly constructs a net/http.Server, which tightly couples the subscriber to a single HTTP server implementation.

While this works well today, it makes it harder to extend the subscriber to support alternative server setups and customise server behaviour without modifying core subscriber logic.

Details

This PR introduces a small abstraction to decouple server creation while keeping backward compatibility.

Alternative approaches considered (if applicable)

Exposing http.Server directly in config. Avoided because

1. Server address currently passed in subscriber constructor would be duplicated in server construction causing confusion. Avoiding this would break backward compatibility.
2. Ownership of net/http.Server becomes unclear.

Checklist

• I wrote tests for the changes.
• All tests are passing.
  • If you are testing a Pub/Sub, you can start Docker with make up.
  • You can start with make test_short for a quick check.
  • If you want to run all tests, use make test.
• Code has no breaking changes.
• (If applicable) documentation on watermill.io is updated.
  • Documentation is built in the github.com/ThreeDotsLabs/watermill/docs.
  • You can find development instructions in the DEVELOP.md.