Adds mounts to fly backend (Continuation #22 PR)

[pxp9]

This PR is intended to continue @benbot work in #22 .

This PR takes his changes and integrates it, to the current library state.

Feel free to add some comments or suggestions.

[pxp9]

It seems @benbot is interested in this feature,

@SteffenDE is it possible to review? Sorry to ping , it is because of the long time.

[SteffenDE]

@pxp9 @benbot fwiw I complained to Chris at ElixirConf that to an outside observer flame could look like a dead project. He insisted that that's not true and that he's aware of the volume mounts feature requests / PRs, but he thinks that it's not a good idea (I don't know any more details there).

It would be best if he could answer himself (cc @chrismccord).

That said, I am not maintaining flame. So while I'm part of the phoenixframework org, I never used flame and I don't know anything about the code. So even though I want to help move this along, I'm not the right person to review. I pinged Chris about this, so let's see if we can get a response :)

[chrismccord]

I'm not convinced we want to support volumes at this time as it requires coordination across servers and the fly backend (and flame) has intentionally avoided adding such cross node coordination. Thanks!