Add utilily pytest fixtures for IntentGuard and IntentGuardOptions

[ELC]

As this tools integrates with pytest, it would be great to have the following fixtures made available automatically:

import intentguard as ig

import pytest

@pytest.fixture
def guard_options() -> ig.IntentGuardOptions:
    return ig.IntentGuardOptions(
        num_evaluations=1,
        temperature=0.4,
    )


@pytest.fixture
def guard(guard_options: ig.IntentGuardOptions) -> ig.IntentGuard:
    return ig.IntentGuard(guard_options)

This will allow to refactor tests from

import intentguard as ig

def test_intention() -> None:
    guard = ig.IntentGuard()
    guard.assert_code(intention, {"module": imported_module })

To:

import intentguard as ig

def test_intention(guard: ig.IntentGuard) -> None:
    guard.assert_code(intention, {"module": imported_module })

The user could do this anyway for their codebase, but as other tools like VCR, request-mocks, pytest-subtest, syrupy do, I believe it is convient and enhances the developer experience.