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/doc.go

@@ -0,0 +1,3 @@
+// Package agentproc contains logic for interfacing with local
+// processes running in the same context as the agent.
+package agentproc