Add option to configure template job HCL file

Previously, the template job HCL file was hardcoded using go:embed
in the binary. However, this did not allow users running Poseidon
to change its content. Now, users can change the content of the
template job HCL file using the configuration option.

configuration.example.yaml

@@ -16,6 +16,8 @@ server:
     keyfile: ./poseidon.key
   # If true, an additional WebSocket connection will be opened to split stdout and stderr when executing interactively
   interactiveStderr: true
+  # If set, the file at the given path overwrites the default Nomad job file in internal/environment/template-environment-job.hcl
+  templateJobFile: ./poseidon.hcl
 
 # Configuration of the used Nomad cluster
 nomad: