Force stop/remove leaves `lima-driver-*` processes running

[unsuman]

Force stopping or removing instances currently leaves external driver processes lima-driver-* running as zombies on the system. We should add cleanup logic in driver/external/server.go to track external driver PIDs and implement a signal termination function.

[unsuman]

Is it a good idea if we create driver.pid inside .lima/{INSTANCE}/ to store the external driver's PID? Since it is used here which might be confusing:

lima/pkg/limatype/lima_instance.go

Line 43 in 8b72891

DriverPID int `json:"driverPID,omitempty"`