[NEW] Module support for Sentinels

[dmitrypol]

The problem/use-case that the feature addresses

Modules allow to extend core Valkey Server with special feature that do not make sense to be in the core product. But Sentinels do not support modules.

Description of the feature

Update Valkey Sentinel to include valkeymodule.h (and other code changes). This will enable Sentinel users to build modules they need to extend Sentinels.

Additional information

One use case is to better integrate Sentinels into various monitoring systems that companies are already using.