[Refactor] Move MatchingLabels to association.go [1/N]

[rueian]

Following #2045

We like to move all client.MatchingLabels{utils.RayClusterLabelKey: string} to the association.go, so that we can maintain consistency in the association methods by not scattering MatchingLabels everywhere.

Including:

kuberay/ray-operator/controllers/ray/raycluster_controller.go

Line 484 in 9068102

filterLabels := client.MatchingLabels{utils.RayClusterLabelKey: instance.Name}

kuberay/ray-operator/controllers/ray/raycluster_controller.go

Line 515 in 9068102

filterLabels := client.MatchingLabels{utils.RayClusterLabelKey: instance.Name}

kuberay/ray-operator/controllers/ray/raycluster_controller.go

Line 1283 in 9068102

filterLabels := client.MatchingLabels{utils.RayClusterLabelKey: newInstance.Name}

[fscnick]

Hi @rueian, I'd like to help with this issue.