Missing Syscall Signals like SIGRTMIN/SIGRTMAX

**Dependency Follow**
[Hashicorp Nomad Client](https://github.com/hashicorp/nomad/blob/main/client/fingerprint/signal.go) ---> [consul-templates](https://github.com/hashicorp/consul-template/blob/main/signals/signals.go)

**Problem**
[Hashicorp Nomad Client](https://github.com/hashicorp/nomad/blob/main/client/fingerprint/signal.go) is using this project to discover Unix OS signal via [consul-template/blob/main/signals/signals.go](https://github.com/hashicorp/consul-template/blob/main/signals/signals.go) and [Consul Templates Unix Signals](https://github.com/hashicorp/consul-template/blob/main/signals/signals_unix.go).

In my use-case where I am running [Apache Impala](https://impala.apache.org/) which is using `SIGRTMIN` as [graceful shutdown signal](https://github.com/apache/impala/blob/master/bin/graceful_shutdown_backends.sh#L45). I can't send this signal via Hashicorp Nomad Job since its not being discovered by Client and Hashicorp Client doesn't implement any specific logic to find supported signals.

**PR Request**
If someone have knowledge about OS Signals and Golang, please add this support in this project so it can be automatically being added to Hashicorp Nomad.

Start by reading consul-template/signals/signals.go and signals_unix.go, then compare how Hashicorp Nomad discovers Unix signals through those entry points. Verify how SIGRTMIN and SIGRTMAX should be represented for the Apache Impala shutdown use case, and confirm that Nomad can discover and send them when the work is complete.

Written by the indexing model from the issue text.