Mounts per process group for Apps v2

@dangra appreciate the work and info here!

I’m sure I’m echo’ing a few others before, but it would be great to share one volume between two processes.

The main use case I’m thinking about is being able to point ActiveStorage (Rails) to a singular fly mounted volume, where images/data can be accessed in both web and worker processes.

ActiveStorage can only be pointed to one source at a time per environment, therefore distinct volumes between processes undermines the ability to use the library.

It’d also be great to not have to use a third party storage solution, like S3.