[litertlm] Refactor Python API with ABC and simplify usage pattern.

[copybara-service]

[litertlm] Refactor Python API with ABC and simplify usage pattern.

This change refactors the LiteRT LM Python API to provide a more idiomatic and
simplified interface for users. Key changes include:

• Abstract Base Classes (ABC): Introduced AbstractEngine and AbstractConversation
  in a new interfaces.py module. The C++ implementation classes are registered as
  virtual subclasses, allowing for proper inheritance checks (e.g.,
  isinstance(engine, AbstractEngine)) across the C++/Python boundary.
• Simplified Engine & Conversation Lifecycle:
  • Users can now directly instantiate litert_lm.Engine with configuration
    parameters, which internally handles ModelAssets and EngineSettings.
  • Added engine.create_conversation() to eliminate the need for manual
    ConversationConfig and Conversation.create calls.
• send_message and send_message_async now support both str and dict inputs. String inputs are automatically wrapped into a user-role message.
• Moved the Backend enum to Python (interfaces.py).
• Internal Refactoring: Updated C++ bindings to support the new factory methods and
  simplified logging initialization using a mapping-based approach.
• Enhanced Testing: Updated existing tests to the new API and added new test cases
  for ABC inheritance, simplified conversation creation, and string input support.