Ability to define the service id for Raven client

Instead of generating a service id with a big ugly sha1 inside (like `monolog.raven.client.4b9d0f59c45d393c0fc2ac65d6f07ff1b71954cd`) to define the Raven client service, we can define a custom service id.

To register some error handler on the Raven client, we need to manually define the service.
And I found more easily/readable to define the service with a custom name instead of using and ugly sha1 one.