[flagd-ui] add back legacy REST APIs to empower programmatic usage

[dottorblaster]

Changes

Since we switched to Elixir, Phoenix and LiveView for the FlagD UI we lost the ability to programmatically interact with the API of the service to script behaviors. Since we got a couple feedbacks about that, this PR restores the REST APIs that were removed in the rewrite process due to not being necessary anymore thanks to LiveView.

Merge Requirements

For new features contributions, please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[julianocosta89]

hey @dottorblaster this looks interesting.
Could you add an example to the readme on how to use it?

I think it would also be worth adding something to the OTel.io docs