Enhance savepoint related features and reorganize those routines

[elanv]

Currently, savepoint and its related routines are scattered in several places. I was also involved in developing those routines. But I think it would be nice if those were organized more to make this operator easier to maintain and extend. Therefore I had made the PR GoogleCloudPlatform/flink-on-k8s-operator#420.

What do you think about the PR? @regadas
If you agree I will try to rework the PR based on this fork.

[regadas]

Hi @elanv I had a quick look to the PR and it looks very interesting! Feel free to make a PR and rebase on this fork; There are quite some changes including the addition of flinkVersion.

[elanv]

Thanks for the review. @regadas
It will take some time, but let's proceed.