Client termination grace period should be configurable

Currently there is a hard-coded grace period of 2 seconds for a client to terminate, but some clients may require a longer period to cleanly shut down. The grace period should be configurable as a client option.

Current period is set at https://github.com/hashicorp/go-plugin/blob/586d14f3dcef1eb42bfb7da4c7af102ec6638668/client.go#L560