feat(BA-2753): Spawn multiple agents and route RPC appropriately

This change adds support for actually spawning multiple agents within
the same agent server and adding agent_id field for all appropriate RPC
calls in the agent server, then ensuring that the manager sends that
info such that the agent server can correctly route the RPC calls to the
correct agent.

Author: hhoikoo

configs/agent/sample.toml

@@ -24,7 +24,7 @@
   # Scaling group type
   scaling-group-type = "compute"
   # Allowed compute plugins
-  ## allow-compute-plugins = [ "ai.backend.activator.agent", "ai.backend.accelerator.cuda_open",]
+  ## allow-compute-plugins = [ "ai.backend.accelerator.cuda_open", "ai.backend.activator.agent",]
   # Blocked compute plugins
   ## block-compute-plugins = [ "ai.backend.accelerator.mock",]
   # Allowed network plugins
@@ -376,7 +376,7 @@
     # Scaling group type
     scaling-group-type = "compute"
     # Allowed compute plugins
-    ## allow-compute-plugins = [ "ai.backend.activator.agent", "ai.backend.accelerator.cuda_open",]
+    ## allow-compute-plugins = [ "ai.backend.accelerator.cuda_open", "ai.backend.activator.agent",]
     # Blocked compute plugins
     ## block-compute-plugins = [ "ai.backend.accelerator.mock",]
     # Allowed network plugins