detect system certificates by default

[dethmasque]

It is possible to use the --insecure option when adding a / deploying to a server that utilizes system certificates, but we should detect them automatically and may do this by adding a call to ssl.create_default_context here: https://github.com/rstudio/rsconnect-python/blob/master/rsconnect/http_support.py#L98

Note: If a server was added using the --insecure option we try and persist that setting in the server metadata.