Fixes #106

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Co-authored-by: Javier Viola <javier@parity.io>

* wip, refactor provider trait

* fmt

* wip

* wip, add from_config to NetworkSpec

* config reorgs

* add logic for create NetworkSpec

* feat: refactored FileSystem trait and errors

* chore: removed unused local_file, stdout will be hardcoded in provider at the moment

* feat: moved MockFileSystem to InMemoryFileSystem, files too, and added mirror implementation of tokio::fs with tests

* feat: added new method append to FileSystem trait

* feat: implemented new append method on InMemoryFileSystem

* feat: added anyhow dependencies to support crate

* feat: refactored FileSystem trait to expose only a single wrapped error

* feat: refacto InMemoryFileSystem following FileSystem trait refacto

* feat: added conversion between io::Error and FileSystemError

* feat: added implementation of LocalFileSystem using tokio::fs

* feat: added nix crate dependency to workspace and provider crate

* feat: added uuid with v4 as dev-depen...