Runtime HTTP client resolution in OSGi

Today, HTTP client resolution happens at runtime using Java's [ServiceLoader](https://docs.oracle.com/javase/7/docs/api/java/util/ServiceLoader.html) class. This works fine in most cases, but not in OSGi: https://blog.osgi.org/2013/02/javautilserviceloader-in-osgi.html

We should create an OSGi-aware implementation of [SdkHttpServiceProvider](https://github.com/aws/aws-sdk-java-v2/blob/4413bfda7b77ced3ce9dc43d3cf5f8ddca2495e2/core/sdk-core/src/main/java/software/amazon/awssdk/core/internal/http/loader/SdkHttpServiceProvider.java), and make current and future implementations of the HTTP client SPI loadable in an OSGi environment.