[inputs.gnmi] Support Arista EOS origin=cli

### Use Case

Arista EOS supports origin=cli where the path becomes the command you'd like to execute. I'd like to subscribe to the cli origin and execute commands at a sample interval to generate metrics. See command output below.

```bash
[admin@eos-switch-1 ~]$ gnmi -username admin -password admin -addr localhost:50000 get origin=cli "/show ip route vrf all summary"
/show ip route vrf all summary:
{
"aggregate": 0,
"attached": 3,
"bgpCounts": {
"bgpExternal": 65,
"bgpInternal": 1,
"bgpLocal": 0,
"bgpTotal": 66
},
"connected": 3,
"dynamicPolicy": 0,
"gribi": 0,
"internal": 7,
"isis": 0,
"isisCounts": {
"isisLevel1": 0,
"isisLevel2": 0,
"isisTotal": 0
},
"maskLen": {
"31": 3,
"32": 74,
"8": 2
},
"totalRoutes": 79,
"vcs": 0
}
```

### Expected behavior

Ignore path validation when origin is set to cli. Subscribe to the path as provided. If it fails, it fails. The name tag should be the path/command executed.

### Actual behavior

Telegraf currently returns the following error:

```bash
[telegraf] Error running agent: starting input inputs.gnmi: invalid string path /show ip route vrf all summary: invalid node name: "show ip route vrf all summary"
```

Which is I believe is raise from here: https://github.com/google/gnxi/blob/master/utils/xpath/xpath.go#L214

### Additional info

_No response_

Start at inputs.gnmi and trace its path validation to the google/gnxi xpath/xpath.go validation cited at line 214. Verify that origin=cli subscribes to the supplied command path without validation, preserves failures, and uses the executed path as the name tag.

Written by the indexing model from the issue text.