Add a file storage to plugin registry

This MR adds a simple file store to the plugin registry.

Description

The file store allows a very basic and simple persistence of plugins. The aim is to provide a simple implementation that allows plugin information to be available even after a restart of the registry.

Related Issue

Motivation and Context

Currently UUIDs for plugins within the registry have been hard-coded.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

dev_env_data/plugin-registry/plugin-store.json

+[{"id":"e2c358b3-6482-4010-b0d8-679dff73153b","path":"plugins/arista/bundled_plugin.zip","manifest":{"name":"Arista","firmware":"standard","author":"goSDN-Team","version":"1.0.0"}},{"id":"d1c269a2-6482-4010-b0d8-679dff73153b","path":"plugins/openconfig/bundled_plugin.zip","manifest":{"name":"Openconfig","firmware":"standard","author":"goSDN-Team","version":"1.0.0"}}]