Local filesystem storage

[piotr-iohk]

Following with the changes from #9, this pr introduces local filesystem storage implementation in delegation_backend package.

Overview

• uptime-service-backend operator may use local filesystem as storage backend by adding to the app_config.json:

  "filesystem": {
    "path": "/path/to/local/dir"
  }

• as implemented in Prepare configs for db and filesystem #9 there can be only one storage backend, after this pr two are supported: aws, filesystem

Testing

• there was an integration test added that is testing if blocks and submissions are stored on local filesystem in case if app_config.json contains local filesystem config.

[Smorci]

Nothing major, just some minor comments.

[Sventimir]

Looks okay to me.

[Smorci]

Looks good!