Refactor disposables

[victimsnino]

Related to: #666 Attempt to make disposables-related logic a bit more clear:

• user's observer HAVE to has preferred_disposable_mode enum variable to define it's preferred mode: auto, external or none
• observable HAVE to has optimal_disposable_strategy typename to provide optimal strategy for passed callbacks/observers
• chain operator HAVE to has updated_optimal_disposable_strategy typename to provide optimal strategy for passed callbacks/observers with respect to previous disposable_strategy

Summary by CodeRabbit

• Documentation

  • Expanded and refined documentation on Reactive Programming, including core concepts, operators, and resource management.
  • Introduced a new memory model concept for better performance and efficiency.

• New Features

  • Added new static constants for preferred disposable modes in various observer strategies.
  • Introduced new type aliases for optimal disposable strategies across different classes.

• Bug Fixes

  • Updated method signatures to improve disposable handling in observers.

• Refactor

  • Renamed and restructured several classes and type aliases for consistency and clarity in disposable management.
  • Enhanced the naming conventions related to disposables for better alignment across the codebase.
  • Simplified the class hierarchy for disposable management.
  • Updated the default container type for managing disposables in the composite disposable class.