proxy: add support for request proxies

[xvaier]

No description provided.

[aviau]

Actually now that I read the code and that I see that we can pass a pre-configured Session object, I don't think this is worthy of changing the API of the client.

Just use the client like this:

session = FlareApiClient._create_session()
session.proxies.update(...)
client = FlareApiClient(session=session)

This avoids adding anything to the API surface and covers your (quite niche) use case. We should avoid adding new arguments for everything that Session already supports. The goal of having a session argument is exactly so that people can do this.