Add a timeout environment variable

https://github.com/rstudio/rsconnect-python/blob/3c8e1a1bc6b04e89ee585f385c9b3bf74740f448/rsconnect/api.py#L61

We have a customer who has a pretty slow proxy (VPC) between their deployment environment and Connect  and want some way to make the hardcoded timeout configurable and accessible.