Support customized attributes for CpuController and MemController

Customized attributes are useful for customized kernels.

Usage:
	let resource = &mut cgroups::Resources::default();
	resource.cpu.attrs.insert("cpu.cfs_init_buffer_us", "10".to_string());
	// apply here

Signed-off-by: Tim Zhang <tim@hyper.sh>

src/memory.rs

@@ -21,8 +21,8 @@ use crate::events;
 use crate::flat_keyed_to_hashmap;
 
 use crate::{
-    ControllIdentifier, ControllerInternal, Controllers, MaxValue, MemoryResources, Resources,
-    Subsystem,
+    ControllIdentifier, ControllerInternal, Controllers, CustomizedAttribute, MaxValue,
+    MemoryResources, Resources, Subsystem,
 };
 
 /// A controller that allows controlling the `memory` subsystem of a Cgroup.
@@ -834,6 +834,8 @@ impl ControllIdentifier for MemController {
     }
 }
 
+impl CustomizedAttribute for MemController {}
+
 impl<'a> From<&'a Subsystem> for &'a MemController {
     fn from(sub: &'a Subsystem) -> &'a MemController {
         unsafe {