Enhance Resolver Service with SPI Support

[laeubi]

Enables obtaining resolver instances outside OSGi frameworks using Java SPI, similar to FrameworkFactory

Problem

Resolvers are needed in many scenarios before/outside OSGi:

• Framework initialization
• Build tools (Maven, Gradle, bnd)
• IDEs analyzing dependencies
• Provisioning tools
• Feature launchers

Currently requires referencing specific implementation classes.

Solution

New ResolverFactory interface using Java Service Provider mechanism, plus osgi.resolver.class framework property for resolver selection.

Changes

• ResolverFactory.java: New interface for creating resolvers
• Constants.java: Added FRAMEWORK_RESOLVER_CLASS constant
• service.resolver.xml: New "Obtaining Resolvers Outside OSGi" section
• framework.lifecycle.xml: Added osgi.resolver.class property documentation

Usage

ServiceLoader<ResolverFactory> loader = 
    ServiceLoader.load(ResolverFactory.class);
Resolver resolver = loader.findFirst()
    .map(ResolverFactory::getResolver)
    .orElseThrow();

Benefits

• Standard, discoverable way to obtain resolvers
• Frameworks expose resolvers for tooling
• Same resolution algorithm in dev and runtime
• Pluggable resolver implementations
• Optional logging for debugging

[laeubi]

@tjwatson I (hopefully) addressed your review comments now that is:

1. Using Resolver as the SPI interface directly, no factory
2. Removing any changes in the core spec, focus on Resolver spec only.