[counterpoll] make the 'eni' command functional only on the DPU

[Yakiv-Huryk]

What I did

To make the 'eni' command only appear in the list of commands when the 'switch_type' is 'dpu'.
This way the "counterpoll --help" output shows only what is supported on the system.

Current output (contains "eni" while the switch_type is not "dpu"):

admin@sonic:~$ counterpoll
Usage: counterpoll [OPTIONS] COMMAND [ARGS]...

  SONiC Static Counter Poll configurations

Options:
  --help  Show this message and exit.

Commands:
  acl               ACL counter commands
  config-db         Config DB counter commands
  eni               ENI counter commands
  flowcnt-route     Route flow counter commands
  flowcnt-trap      Trap flow counter commands
  pg-drop           Ingress PG drop counter commands
  port              Port counter commands
  port-buffer-drop  Port buffer drop counter commands
  queue             Queue counter commands
  rif               RIF counter commands
  show              Show the counter configuration
  tunnel            Tunnel counter commands
  watermark         Watermark counter commands

Output after this change:

admin@sonic:~$ counterpoll
Usage: counterpoll [OPTIONS] COMMAND [ARGS]...

  SONiC Static Counter Poll configurations

Options:
  --help  Show this message and exit.

Commands:
  acl               ACL counter commands
  config-db         Config DB counter commands
  flowcnt-route     Route flow counter commands
  flowcnt-trap      Trap flow counter commands
  pg-drop           Ingress PG drop counter commands
  port              Port counter commands
  port-buffer-drop  Port buffer drop counter commands
  queue             Queue counter commands
  rif               RIF counter commands
  show              Show the counter configuration
  tunnel            Tunnel counter commands
  watermark         Watermark counter commands

How I did it

Added a register_dynamic_commands() to dynamically add the 'eni' command.
Updated the unit tests.

How to verify it

Run the counterpoll_test.py test

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[651477787]

fine, approve

[651477787]

approved