Extensible Process Control API (pid and Signals)

[guptapratykshh]

currently, fs2.io.process.Process wraps Java’s Process entirely but only exposes five basic methods (isAlive, exitValue, stdin, stdout, stderr). Java 9+ heavily expanded process management, but fs2 actively hides these new capabilities behind its minimal abstraction. Users frequently need to write out Process IDs to .pid files, or they want manual, fine-grained control to gracefully terminate specific child tasks without canceling the parent Resource.

We can expand the Process[F] trait with:
def pid: F[Long] (Fetch the OS process ID)
def terminate: F[Unit] (Send graceful SIGTERM)
def kill: F[Unit] (Send absolute SIGKILL)