feat: implement agent process management (#9461)

- An opt-in feature has been added to the agent to allow
   deprioritizing non coder-related processes for CPU by setting their
   niceness level to 10.
- Opting in to the feature requires setting CODER_PROC_PRIO_MGMT to a non-empty value.

agent/agentproc/syscaller.go

@@ -0,0 +1,19 @@
+package agentproc
+
+import (
+	"syscall"
+)
+
+type Syscaller interface {
+	SetPriority(pid int32, priority int) error
+	GetPriority(pid int32) (int, error)
+	Kill(pid int32, sig syscall.Signal) error
+}
+
+const defaultProcDir = "/proc"
+
+type Process struct {
+	Dir     string
+	CmdLine string
+	PID     int32
+}