Design pluggable resolvers for custom version sources #25

Summary

Allow advanced users to plug in custom resolvers for internal base images or bespoke version files, extending the action beyond the built-in discovery logic.

Why

Enterprises often mirror images or maintain proprietary version files. A pluggable architecture would let them hook into the action without forking.

What needs to happen

• Define an interface for resolvers (inputs, outputs, error handling).
• Implement a plugin loader that can resolve modules from the action package or a repository path.
• Provide at least one sample custom resolver and documentation.
• Ensure security: sandbox execution and validate outputs.
• Add tests covering both default resolvers and user-supplied ones.

References

• Roadmap v2.0 idea: "Pluggable resolvers".

Acceptance criteria

• Developers can register custom resolvers via configuration.
• Default behavior remains unchanged when no custom resolver is supplied.
• Documentation explains how to write and register a resolver safely.