feat(Spec): add resolver step to spec pipeline

[DerManoMann]

Summary

Add a new resolver step to the spec pipeline.
This gives users a chance to register one or more handlers to resolve classes/types that were not processes by the Assembler at all.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Enhancement (improvement to existing functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor (no functional changes)
• Chore (maintenance, dependencies, CI)

Related Issues

Test Plan

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code where necessary
• I have updated the documentation accordingly
• My changes generate no new warnings
• New and existing tests pass locally
• Any dependent changes have been merged and published