Introduce SslProfile class to simplify SSLService

[tvernum]

This change introduces a new SslProfile class.

The SslProfile is roughly analogous to SslConfiguration and provides the runtime implementation support for an "ssl context" (but here called a "profile" because SSLContext is already a thing).

This means that a number of methods that were previously called on SSLService are now called on a contextual SslProfile.
For example, SSLService.getHostnameVerifier(SslConfiguration) is now SslProfile.hostnameVerifier()

This has two primary benefits

1. Moving these methods out of SSLService provides a cleaner split. SSLService is now (more of) a manager for accessing profiles and configurations, and the profiles provide the runtime support for creating SSL connections.
2. It moves us a step towards having extensible SSL contexts/profiles. Because a SslProfile is self contained, we can inject one into a defined extension point without needing any direct reference to an SSLService. That is we can work towards an SPI extension point such as

   public interface SslExtension {
        public Set<String> getSslProfileNames();
        public void applySslProfile(String name, SslProfile profile);
   }
   

   and that interface doesn't need to know anything more than what is contained in SslProfile (it knows nothing of SSLService).

[elasticsearchmachine]

Pinging @elastic/es-security (Team:Security)

[tvernum]

@slobodanadamovic No rush on this. I'm not likely to have time to come back to it for another couple of weeks anyway.

I tried to make each commit self contained, so if you want to review it commit-by-commit that should work quite well.

[slobodanadamovic]

LGTM 👍

Thanks for breaking refactoring into separate commits! Made it easier to review.