Add an option to enable keepalive

[sentrivana]

Problem Statement

In case folks are experiencing network issues between the SDK and Sentry, we in general recommend tweaking socket_options directly with a predefined set of settings. This seems to alleviate the issue most of the time, but feels rather low-level and hacky.

Solution Brainstorm

Node has a separate keepAlive transport option. We could do something similar for the Python SDK; maybe a top-level enable_keepalive (or enable_keep_alive?) option that sets the socket option to what we usually recommend, while keeping the socket_options around as well for users who need more control over things.

Make sure to also call this new option out in the docs (especially in the Crons part since crons seem to be often affected).