CLI HTTP timeouts and connection limit are configurable per environment

The shared HTTP URLSession used to download and upload CLI artifacts is now tunable through TUIST_HTTP_TIMEOUT_INTERVAL_FOR_RESOURCE (default 90s), TUIST_HTTP_TIMEOUT_INTERVAL_FOR_REQUEST (default 120s), and TUIST_HTTP_MAXIMUM_CONNECTIONS_PER_HOST (default 20). Self-hosted and constrained-network operators can raise the resource timeout so a recoverable-but-slow download finishes as a cache hit instead of falling back to a from-source build. The defaults are unchanged when the env vars are unset.