Epic: Implement Backstage plugin

[emyller]

Context: #2145

Goal

We want to add Flagsmith to the Backstage plugins directory with basic functionality.

Scope

• Initialize a micro React app as a new Backstage plugin, learn how.
  • Check examples shared in the reference issue: #2145
  • A Backstage plugin seems to have pre-defined UI style and components.
  • Examples are out of date - check latest Backstage frontend framework.
• Implement feature flag read-only functionality:
  • List feature flags and [only] display overrides and other useful information.

Variations and segments overrides should be displayed similarly to the Jira integration:

Other functionality should be easily accessible via links to the Flagsmith dashboard.

No goals