Allow setting keep_alive via environment variable.

[rodolfoBee]

Problem Statement

Currently it is not possible to configure keep_alive using an environment variable. This can be an issue when users that use AWS layer that can only be configured using environment variable encounter connection issues like this #4311 where this option can help.

Solution Brainstorm

Add support for an environment viable, such as SENTRY_KEEP_ALIVE, to set keep alive to true.

[szokeasaurusrex]

Good idea! So that we can unblock #4311, I will set a higher priority on this issue