SAMZA-1542 refactor config classes in samza-core java to hide non-necessary interfaces

[fredji97]

With previous inheritance, it exposed everything from Config class, which is a bad practice and is error prone. We are refactoring it to use composition over inheritance make it more robust.

[sborya]

I think it is ok. But we need to be careful with methods like getFilteredConfig(). We need to make sure (probably add it as a release note) that no module is using configs not prefixed correctly for this module.

[vjagadish1989]

Hey @fredji97 ,
Thanks for this PR. While I'm +1 to your idea, I'm trying to ensure that the open-source codebase doesn't remain in a state where some Config classes follow inheritance while others follow composition for a long period of time. Ideally, I would love to make all changes in one-shot throughout the Samza code-base. It need not have to be a single PR but would be nice if you have all necessary PRs ready and if they can be merged in quick succession. cc: @prateekm @nickpan47