Add support for a `FeatureFlagService` for LaunchDarkly

[pBouillon]

Issue Summary

Add support for a FeatureFlagService for LaunchDarkly.

Category

- [ ] Bug report
- [ ] Regression
- [x] Feature suggestion
- [ ] Documentation issue
- [ ] Other

Expected Behavior

The FeatureFlagService should be able to get feature flags from LaunchDarkly.

Actual Behavior

Currently, there is FeatureFlagService implementation for this provider, and users need to create their own implementation

Steps to Reproduce

N/A

Environment

• Browser/Device: N/A
• Node version: N/A
• Version: N/A

Additional Information

LaunchDarkly is a popular feature flagging service that enables real-time feature flagging in applications. Adding support for LaunchDarkly in the FeatureFlagService would make it easier for potential users to transition to ngx-flagr.

The FeatureFlagService should be configurable in the provider with options, including the API Key, that would be essential to integrating with LaunchDarkly.

It might be beneficial to move this implementation to a dedicated package, such as@ngx-flagr/providers, and create a new one under @ngx-flagr/providers/launch-darkly

Other package names could be:

• @ngx-flagr/providers
• @ngx-flagr/integrations
• @ngx-flagr/wrappers