http: add maxTotalSockets to agent class

Add maxTotalSockets to determine how many sockets an agent can open.
Unlike maxSockets, The maxTotalSockets does not count by per origin.

PR-URL: https://github.com/nodejs/node/pull/33617
Fixes: https://github.com/nodejs/node/issues/31942


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>