switch to httpx2

The `httpx` library is unmaintained, on of the core maintainers decided to fork it: https://github.com/pydantic/httpx2

For the now the version is still compliant with the original httpx so we can just do `import httpx2 as httpx`

No files or tests are named. Start by locating the dependency declaration and all Python imports in stac-auth-proxy, then verify that the proposed httpx2 alias remains compatible with current usage. Done means the dependency and imports use httpx2 without changing proxy authentication behavior.

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