Implement JavaScript and Python Execution Components

This PR adds JavaScript and Python run-time features to the golem:exec interface as requested in issues document. Closes #33

/claim https://github.com/golemcloud/golem-llm/issues/33

What’s Done

Language Support:

• JavaScript: ES5 through ES2022
• Python: versions 3.8 through 3.12

Key Features:

• Streaming execution - get output as code runs, not just when finished
• Session management - upload files, run code, download result
• Resource limits - enable timeouts and memory limits
• File handling - UTF-8, Base64, and Hex encoding enabled
• Error handling - short error messages for different failure modes

How It Works

1. Upload your code files to a session
2. Set the execution limits (timeout, memory)
3. Run code with streaming or normal execution
4. Return results through stdout, stderr, and exit codes

Testing

• Tests cover streaming, timeouts, file operation, and language checks
• Works in both native and WASM modes
• Integration tests check the end-to-end pipeline

Files Changed

New Components:

• exec/exec/ - Central executor (1,200+ lines)
• exec/exec-javascript/ - JavaScript context (400+ lines)
• exec/exec-python/ - Python context (400+ lines)

Key Files:

• executor.rs - Core execution logic
• runtime/ - Runtime factories for each language
• stream.rs - Streaming output handling
• WIT files defining the interfaces