feat: Implement Python V2 X402Client class

[marcosmmb]

Description

This PR implements V2 X402Client for Python by following the TypeScript reference implementation and the proposed V2 Changelog.

Related to https://github.com/coinbase/x402/pull/482/

As per the TODO guidance, I have added new x402.core, and x402.client modules. Because there's already a V1 types.py file, I have added the new types module as x402.core.types.

Tests

============================================================================================================================== test session starts ===============================================================================================================================
platform linux -- Python 3.10.6, pytest-8.4.0, pluggy-1.6.0
rootdir: /root/x402/python/x402
configfile: pytest.ini
plugins: asyncio-1.0.0, anyio-4.9.0
asyncio: mode=auto, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 104 items                                                                                                                                                                                                                                                              

tests/client/test_x402_client.py .................                                                                                                                                                                                                                         [ 16%]
tests/clients/test_base.py .......                                                                                                                                                                                                                                         [ 23%]
tests/clients/test_httpx.py .........                                                                                                                                                                                                                                      [ 31%]
tests/clients/test_requests.py ..........                                                                                                                                                                                                                                  [ 41%]
tests/fastapi_tests/test_middleware.py ............                                                                                                                                                                                                                        [ 52%]
tests/flask_tests/test_middleware.py ............                                                                                                                                                                                                                          [ 64%]
tests/test_common.py ......                                                                                                                                                                                                                                                [ 70%]
tests/test_encoding.py ...                                                                                                                                                                                                                                                 [ 73%]
tests/test_exact.py ......                                                                                                                                                                                                                                                 [ 78%]
tests/test_paywall.py ..............                                                                                                                                                                                                                                       [ 92%]
tests/test_types.py ........                                                                                                                                                                                                                                               [100%]

============================================================================================================================== 104 passed in 0.76s ===============================================================================================================================

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

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