feat: Implement multi-registry support with separate feature modules

[finxo]

PR's key points

• Introduces multi-registry support allowing separate feature modules to generate their own registries
• Adds MiniRegistry interface and refactors Mini class to support multiple registries discovery
• Creates two sample feature modules (sample-counter-feature and sample-message-feature) demonstrating isolated registries
• Updates documentation with comprehensive multi-module setup guide and bootstrap patterns
• Adds MultiRegistrySampleActivity to showcase runtime linking of separate feature registries
• Maintains backward compatibility with legacy single registry approach

How to review this PR?

1. Start with the documentation updates in README.md to understand the multi-registry concept
2. Review the new MiniRegistry interface and Mini.link() implementation
3. Examine the sample feature modules and how they generate separate registries
4. Test the MultiRegistrySampleActivity in the sample app to see the feature in action
5. Verify integration tests pass: ./gradlew :mini-processor-multiregistry-test:test

Related Issues (delete if this does not apply)

Definition of Done

• Tests pass
• Works with Proguard
• There is no outcommented or debug code left