MetalOS compatibility

Summary:
Make `fb-oomd` and `fb-oomd-cfgen` MetalOS compatible:

- Generate `oomd2.json` file in the /etc directory in case of MetalOS mode
- Override cgroups slice with `hostcritical.slice`
- Set `/run/oomd` as SystemdRuntime directory
- Create `/run/oomd/dropin` before service starts
- Enable basic health-check using `/metalols/bin/oomd -d` which connects to running `oomd` over /run/oomd/oomd.socket and dumps some stats.
- Set `ProtectControlGroups` to `False`.

**Follow up:**

Figure out what to do with the `fb_cgroups` settings:

```
# cat /etc/systemd/system/fb-oomd.service.d/fb_cgroups_slice_override.conf
# This file is maintained by Chef. Do not edit, all changes will be
# overwritten. See fb_cgroups/README.md

[Service]
MemoryLow=64M
OOMScoreAdjust=-1000
MemoryMin=64M
Slice=hostcritical.slice
```

They are applied by Chef and are active on an aliased service but we need to have a parity for them in MetalOS as well.

Reviewed By: chengxiong-ruan

Differential Revision: D59024999

fbshipit-source-id: 22986443fc0be86793b3ac5f242c63386481b3f2

src/oomd/cfgen/src/cfgen.rs

@@ -1046,7 +1046,16 @@ fn get_host_type(node: &Node) -> HostType {
 
 fn main() -> anyhow::Result<()> {
     let mut b = libcfgen::Builder::new();
-    b = b.dynamic_json("oomd2.json", oomd_json);
+    b = b.dynamic_json(
+        if libcfgen::context()?
+            .is_some_and(|c| c.consumer() == libcfgen::ConfigConsumer::METALOS_WDS)
+        {
+            "etc/oomd2.json"
+        } else {
+            "oomd2.json"
+        },
+        oomd_json,
+    );
     b = b.dropin(|node| Ok(oomd_dropin(node)));
     b.run()
 }