Add Starred operator class for representing variable-length arguments

Pull Request description

This PR implements the Starred class to represent starred unpacking expressions in Python (e.g., *args, *items). This feature enables proper AST representation of variable-length arguments in function definitions and calls, as well as sequence unpacking operations.

Resolves: #203

How to test these changes

• Run the tests with pytest libs/astx/tests/test_operators.py::test_starred*
• Create a simple script that uses the Starred operator:

  from astx.base import Identifier
  from astx.operators import Starred
  from astx.variables import Variable
  
  # Example with variable
  args_var = Variable(name="args")
  starred_args = Starred(value=args_var)
  print(starred_args)  # Should output: Starred[*](args)
  
  # Example with identifier
  ident = Identifier(name="items")
  starred_items = Starred(value=ident)
  print(starred_items)  # Should output: Starred[*](items)

Pull Request checklists

This PR is a:

• bug-fix
• new feature
• maintenance

About this PR:

• it includes tests.
• the tests are executed on CI.
• the tests generate log file(s) (path).
• pre-commit hooks were executed locally.
• this PR requires a project documentation update.

Author's checklist:

• I have reviewed the changes and it contains no misspelling.
• The code is well commented, especially in the parts that contain more
  complexity.
• New and old tests passed locally.

Additional information

ASCII representation of the Starred node:

Starred[*]
   |
   +-- value: Variable or other DataType

The implementation follows the established patterns in the codebase:

• Uses proper typing annotations
• Includes thorough testing
• Maintains compatibility with existing AST nodes

This node is essential for representing Python's variable-length argument syntax and unpacking operations in function calls, function definitions, and sequence operations.