Figure out cache configuration

[sipsma]

Questions to be answered:

1. How is cached configuration specified? Is it builtin to the core api, something that clients configure? Can it be configured through other actions (extensions)? Can runtimes declare cache sources themselves in addition to clients?
2. Can/How do you configure "global" cache imports/exports for action execution (as in, cache that applies to the action being invoked and every transitive dependency)
3. Can/How do you per-action cache imports/exports (as in, cache config that only applies to certain actions)?

[sipsma]

My comment here about multiplatform config may be relevant here too: #3064

[sipsma]

There's another issue here tracking this, can centralize on that one: #3422