refactor: integrate cache parameter into setDiscovery() method

[xentixar]

This PR implements the API improvement suggested by CodeWithKyrian in PR #39.

Changes

• Integrate cache into setDiscovery(): Add cache as 4th parameter to setDiscovery() method
• Rename property: Change cache to discoveryCache for clarity
• Remove separate method: Remove the standalone setCache() method
• Update example: Update the cached discovery example to use the new API

Benefits

• Cleaner API: Cache is now clearly associated with discovery operations
• More intuitive: Users don't need to call two separate methods
• Self-documenting: The API makes it obvious that the cache is for discovery

Before

Server::make()
    ->setDiscovery(__DIR__, ['.'])
    ->setCache(new Psr16Cache(new ArrayAdapter()))
    ->build();

After

Server::make()
    ->setDiscovery(__DIR__, ['.'], [], new Psr16Cache(new ArrayAdapter()))
    ->build();

This addresses the feedback from the original discovery caching PR to make the API more ergonomic and intuitive.