Implement dummy functions to avoid dummy warnings.

[crova]

Hello, how is it going?
First of all, thanks for the great work.

We recently integrated your library into our application using ConCache as adapter.
The following configuration:

  @ttl :timer.minutes(15)

  use Cache,
    name: :apollo_io,
    adapter: Cache.ConCache,
    sandbox?: Mix.env() === :test,
    opts: [dirty?: false, global_ttl: @ttl]

Yields the following warnings when using Sandbox during tests:

warning: Cache.Sandbox.dirty_get_or_store/3 is undefined or private
  lib/enaia/apollo_io/cache_forwarder.ex:15

warning: Cache.Sandbox.get_or_store/4 is undefined or private
  lib/enaia/apollo_io/cache_forwarder.ex:15

This PR implement some dummy functions to silence those warnings.

Please let me know if you see any issues.

Cheers.

[MikaAK]

Thanks for this, I think a good solution would be to just implement those two functions however 🤔 I think it should be just a quick call in the Agent, what do you think? That way you get to be able to test with those functions as well 😄

[crova]

Thanks for this, I think a good solution would be to just implement those two functions however 🤔 I think it should be just a quick call in the Agent, what do you think? That way you get to be able to test with those functions as well 😄

I definitely went for the lazy solution.
I'll give it a try to see if I can implement those and improve this PR.