Add configurable realm support for token authentication - #163

Background
The token fetching was hardcoded to hit /realms/master/protocol/openid-connect/token, which is a real pain for anyone not using the master realm. In enterprise setups, you typically have realm admins who can only access their specific realm - they don't get master realm privileges.

What this fixes

• Realm admins can now authenticate against their own realm instead of being forced to use master
• Supports the common enterprise pattern where different teams/environments have their own realms
• Removes a major blocker for organisations that follow least-privilege security practices

Implementation
Added a configurable realm parameter that defaults to "master" so existing code keeps working exactly as before. No breaking changes.

// Still works the same way
$keycloak = new Keycloak($baseUrl, $username, $password);

// Now you can also do this
$keycloak = new Keycloak($baseUrl, $username, $password, realm: 'my-realm');

I decided against changing signatures of other methods to keep this PR as non-invasive as possible. I'd leave these major API changes to you, @fschmtt