Add out-of-the-box config handling

[radiohead]

What

While building an app and deploying it to a Kubernetes cluster, I ended up implementing custom config parsing & handling for both the pre-defined config for operator.RunnerConfig and app.SpecificConfig.

I believe this should be simplified and the SDK should provide (and generate) a way to parse & handle at least the pre-defined configs, but ideally also provide helpers for parsing & handling specific configs (for those, devs will still have to define flags / env vars / etc. manually).

Why

To remove friction and reduce the amount of footguns. Having a standard way of parsing configs will also help us with providing generic Jsonnet / Helm libraries for deploying app operators.

[radiohead]

A few extra bits, related to config:

• We should provide a generic logging config and handle it in the SDK
• We should wire up tracing config in operator.Runner (Yes it's global but I think it's OK to always initialize it there by default. If necessary, devs can pass e.g. Enabled: false to disable tracing initialization in the runner).
• operator.RunnerConfig does not expose some fields from simple.AppConfig. I think this is related to Refactor operator Package #454 but in general I think we should reduce the amount of configs we pass around (e.g. just have app.Config and 1 config per runner type, which extends app.Config with runner-specific settings).