text: Title: Add a custom URL type instead of rely on `httpx.URL`
  Body: - We use the `httpx.URL` in the [ProxyConfiguration](https://github.com/apify/crawlee-python/blob/master/src/crawlee/proxy_configuration.py), in the tests and maybe in other places as well.
  - It kind of shot us into the foot in #618.
  - ~I suggest replacing it with our custom data type (either the Pydantic model or data class).~
      - As Honza said, we could probably utilize some 3rd party library, [yarl](https://pypi.org/project/yarl/) seems like a good option.
  - Also please find other occurrences of the `httpx.URL` in the Crawlee and replace them with our custom type.
    - This applies also to tests: `httpbin: URL`
  - Keep in mind, that URLs in the `Request` model need to be serialized as strings.