Add clab.exec parameter to execute commands when starting containers (#2838)

Closes #2806

Author: ipspace

docs/labs/clab.md

@@ -364,6 +364,7 @@ You can also change these *containerlab* parameters:
 * **clab.type** to set node type (used by Nokia SR OS and Nokia SR Linux).
 * **clab.dns** to [configure DNS servers and search domains](https://containerlab.dev/manual/nodes/#dns)
 * **clab.env** to [set container environment](https://containerlab.dev/manual/nodes/#env) (used to [set interface names for Arista cEOS](https://containerlab.dev/manual/kinds/ceos/#additional-interface-naming-considerations))
+* **clab.exec** to execute a list of commands after the container has started. If you want to start shell scripts, use **clab.binds** or **clab.config_templates** to make those scripts available to the container
 * **clab.license** to configure a license file for those platforms that require one
 * **clab.ports** to [map container ports to host ports](https://containerlab.dev/manual/nodes/#ports)
 * **clab.cmd** to [change the command of a container image](https://containerlab.dev/manual/nodes/#cmd).

netsim/providers/clab.yml

@@ -4,7 +4,7 @@
 description: containerlab with Docker
 config: clab.yml
 lab_prefix: "clab" # Containerlab default, set to "" to remove prefix
-node_config_attributes: [ type, cmd, dns, env, license, ports, startup-delay, restart-policy ]
+node_config_attributes: [ type, cmd, dns, env, exec, license, ports, startup-delay, restart-policy ]
 template: clab.j2
 version: 0.69.3
 # Preserve env to allow user to configure PATH
@@ -47,6 +47,7 @@ attributes:
       startup-delay: int
       restart-policy: str
       network-mode: str
+      exec: { type: list, _subtype: str }
   interface:
     name: str
   link: