Require a LoadBalancer for service discovery

What does this MR do?

The service discovery class now requires a load balancer to be passed in, instead of using a default object. This decouples the class from the global proxy/load balancer we currently use. This makes it easier to create multiple service discovery instances, without accidentally using the wrong load balancer.

This MR has been extracted from !68042 (merged).

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.