feat: Block scheduler scaffolding

[owen-d]

Block Builder Package Refactoring

This PR restructures the Block Builder package to improve modularity and testability. The changes establish a clear separation of concerns and lay the groundwork for future enhancements.

Please see the new Architecture.md for diagrams & in depth changes.

Key Changes

Package Restructuring

• Split pkg/blockbuilder into three focused subpackages:
  • types/: Core interfaces and shared data structures
  • scheduler/: Job queue and scheduling logic
  • builder/: Worker implementation and transport layer

Interface Improvements

• Introduced clear interface boundaries between components:
  • Worker: Job processing and status reporting
  • Scheduler: Job distribution and worker management
  • Transport: Communication layer abstraction

Types

• Centralized type definitions in types package

Documentation

• Updated architecture.md with comprehensive package structure
• Added detailed component diagrams
• Documented design principles and interactions

Testing

• Added support for in-memory transport for testing

Followups

• Integrating the existing builder into the worker interface
• Introducing grpc as a transport
• Metrics/o11y for some of the new constructs
• a scheduler implementation using these
• service.Service support for said scheduler

[ashwanthgoli]

lgtm