LoadBalancerRetryContext should not be package-private

[ajschutte]

Describe the bug
The LoadBalancerRetryContext type is passed as a parameter in the LoadBalancerRetryPolicy, which is supposed to be implemented for custom policies. Being package-private makes that rather difficult.

Sample
Any custom implementation of LoadBalancerRetryPolicy outside the LoadBalancerRetryContext package should be able to reproduce the problem.

[OlgaMaciaszek]

Makes sense @ajschutte .