Support SOCKS proxies

[Roguelazer]

If pysocks is available, urllib3 supports SOCKS proxies, but you can't use the urllib3.ProxyManager class and must instead use urllib3.contrib.socks.SOCKSProxyManager. It would be very nice if sentry_sdk.transport.HttpTransport could detect a proxy set to socks:// (or one of the many variants) and Do The Right Thing.

[kenoss]

I vote one. It is great that sentry_sdk itself has SOCKS proxy support (integrated in the class HttpTransport.).

Rationale:

1. There are situations we have to use sentry under SOCKS proxy. E.g., IoT device with restricted network access.
2. I tried to write external class Socks5HttpTransport, but it's not cool to use.

And then, here's some implementation options.

How to designate proxy URL

A. From http_proxy/https_proxy option, like requests and #1050 .
B. From other new option like socks_proxy.

I guess the members of sentry prefer B because #178 (comment) .

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@untitaker @ahmedetefy Could you let us know your thoughts?