add FileSystem provider

[jramos]

This allows for secrets to be read from a local file system, e.g. those mounted via the Kubernetes Secrets Store CSI Driver.

[jramos]

This may actually work better as a stand-alone provider so that default search paths can be configured to make the keys less verbose. Wdyt?

[gordalina]

Thanks for contributing! I think this is a great native provider. What do you mean about the default search paths?

[jramos]

It would be something like this...

config :hush,
  file_system_provider: [
    root_path: "/opt/thenameofyourcompany/secrets"
  ]

config :app, App,
  ssl_certificate: {:hush, FileSystem, "cert.pem"}

config :app, App.Other,
  ssl_key: {:hush, FileSystem, "private/cert.pem"}

That way "/opt/thenameofyourcompany/secrets" isn't needed in every key, and it would prevent arbitrary paths from being used... all the keys would need to be nested under the configured root_path.

The "search path" idea is similar, but you would have a list of paths, and the directories would be checked in order for the file until one was found.

config :hush,
  file_system_provider: [
    search_paths: [
      "/opt/thenameofyourcompany/secrets",
      "/opt/secrets",
      "/etc/app"
    ]
  ]

[gordalina]

Added a commit, lmk what you think.

[jramos]

Looks great to me!

[gordalina]

Released in v1.2.0 thanks!