Add timeout parameter to addprocs function

Currently, the worker timeout for `addprocs` can only be configured via the `JULIA_WORKER_TIMEOUT` environment variable. It would be more convenient to have a `timeout` keyword argument directly in the `addprocs` function. **Proposed API:** ```julia addprocs(4; timeout=120.0) # 120 second timeout ``` This would be more ergonomic than having to set environment variables before calling `addprocs`.