Allow to specific python_version in gazelle generated py_binary and py_test rules

# 🚀 feature request

### Relevant Rules

### Description

Currently I am using bazel and gazelle to manage a mono repo with multiple python projects.

Some of them using python 3.10 and others are using python 3.11.

When using gazelle with rules_python, I need to manually append `python_version=3.11` to generated py_test rule.

For example

```bazel
# generated by gazelle

py_test(
name = "test_module_a",
srcs = ["module-a/tests/test_module_a.py"],
python_version = "3.11", # manually edit line
deps = [
"//module-a/src/module_a"
],
)
```

### Describe the solution you'd like

We should have a new directive such as `gazelle:python_version` to allow user to override `python_version` attribute in generated `py_test` and `py_binary` rules.

### Describe alternatives you've considered

Gemini also suggested to use a `build_arg` to override default generated rules but it doesn't work.

Start by locating the Gazelle Python generation and directive-handling entry points for generated py_binary and py_test rules. Review how existing directives affect generated attributes, then verify that a python_version directive produces the requested attribute for both rule types without manual edits.

Scritto dal modello di indicizzazione a partire dal testo della issue.