[ENHANCEMENT] Add Support for Custom `HttpRequestExecutor` in `OptimizelyHttpClient`

Description

Currently, the OptimizelyHttpClient in the Optimizely Java SDK uses a default HttpRequestExecutor provided by Apache HttpComponents library for handling HTTP requests. However, there is no extension point to allow developers to supply a custom HttpRequestExecutor.

Adding the ability to inject a custom HttpRequestExecutor would provide greater flexibility, enabling users to replace or extend the default behavior. For example, the inclusion of tools such as the MicrometerHttpRequestExecutor allows the collection of execution metrics, including latency, errors, and throughput.

While upgrading to Apache HttpComponents 5 would modernize the library and bring advanced features, adding this extension point would still be valuable. It ensures that developers can adapt or enhance the HTTP client behavior for monitoring and observability, regardless of which version of Apache HttpComponents is used.

Benefits

• Increased Observability: Developers can use custom executors, like the MicrometerHttpRequestExecutor, to capture valuable execution metrics such as request duration, HTTP status distribution, and network errors. These metrics can be sent to monitoring systems like Prometheus or Datadog.
• Enhanced Extensibility: A configurable HttpRequestExecutor provides an extension point for more advanced use cases, empowering developers to customize the HTTP behavior without requiring changes to the base SDK.
• Future-Proof Design: Introducing this flexibility ensures that users have control over HTTP execution behavior, making it easier to adopt library upgrades in the future, including potential transitions to Apache HttpComponents 5.

Detail

1. Add a configurable property to OptimizelyHttpClient to allow injection of a custom HttpRequestExecutor.
2. Provide a fallback to the existing default HttpRequestExecutor if no custom request executor is supplied.
3. Update documentation to highlight this new extensibility feature and provide examples of using custom executors such as MicrometerHttpRequestExecutor.

Examples

Not right now

Risks/Downsides

Not right now

Start by locating OptimizelyHttpClient and its current default HttpRequestExecutor setup, then review the surrounding HTTP client tests and documentation. Done means a custom executor can be supplied, the existing default remains the fallback, and documentation includes a usage example such as MicrometerHttpRequestExecutor.