feat: add SkillProvider SPI for pluggable skill backends

[danielvolovik96]

Closes #37

Summary

• Add SkillProvider and SkillDescriptor interfaces in the common module as a Service Provider Interface for pluggable skill backends
• Create FileSystemSkillProvider as the default implementation (refactored from existing logic)
• Refactor SkillsFunction to delegate to providers at invocation time
• Add addSkillProvider() to SkillsTool.Builder
• Full backward compatibility with existing addSkillsDirectory/Resource API

Test plan

• Unit tests for FileSystemSkillProvider
• Unit tests for SkillsTool with provider integration
• Verify backward compatibility with existing API

[tbezierslafosse]

Hi, this PR would directly help our use case.

We need skills to be managed through an admin UI, eventually stored in a database, and selected dynamically depending on the current project/context. Users may work on different projects or domains, each requiring a different set of skills, so a static filesystem or classpath-backed source does not fit well.

The proposed SkillProvider looks like the right abstraction for us: expose descriptors from the current backend/context when the tool is built, then lazily load the full skill content when the model invokes it.

A quick review would definitely help !