Enhance httpclientjava provider to use SSLContext created on ECF startup

[scottslewis]

Currently, a customized/customizable SSLContext is created on ECF startup, in org.eclipse.ecf core bundle startup (via fragment org.eclipse.ecf.ssl).

Unlike previous providers, the httpclientjava provider must be passed the SSLContext instance (rather than the SSLSocketFactory) via the HttpClient.Builder during HttpClient instance construction.

[scottslewis]

With pr #138 I've introduced a new osgi service, represented by the service interface class SSLContextFactory this service...available to all ECF filetransfer providers, allows the setting of the provider SSLContext with the implementation of service methods provided by the implementation of the service, rather than through the static methods: SSLContext.getDefault(), SSLContext.getInstance(String) and SSLContext.getInstance(String,String).

The (first) use of this service is in the httpclientjava provider here

This provides flexibility to support needs of httpclientjava provider (and other providers as well).