bug: Flet app built with `flet build web` crashes after failing to find `httpx` and `oauthlib` packages

Flet version: 0.82.0

Problem: both `httpx` and `oauthlib` dependencies are excluded on Pyodide platform, but `authorization_service.py` does eager import of `httpx` and `oauthlib` packages.

Solution: do lazy imports.

Workaround: add this to `pyproject.toml`:

```toml
[tool.flet.web]
dependencies = [
    "httpx",
    "oauthlib"
]
```