feat: add CompilerConfig dataclass for structured compiler options

Replace string-based compiler arguments with a type-safe CompilerConfig
dataclass that provides discoverability and easy customization.

Changes:
- Add CompilerConfig dataclass with fields for common neuronx-cc options
  (lnc, model_type, auto_cast, enable_mixed_precision_accumulation, etc.)
- Add factory methods for_nkipy() and for_nki() with appropriate defaults
- Add get_default_compiler_args() helper to inspect default settings
- Add compiler_config parameter to @baremetal_jit, baremetal_run_traced_kernel,
  and DeviceKernel.compile_and_load()
- Export CompilerConfig and get_default_compiler_args from nkipy.runtime
- Add tutorial section demonstrating CompilerConfig usage

Backward compatible: legacy additional_compiler_args parameter still works.

Author: AoyuQC