[sflow] Added sflow dropped packet notification support

[yehjunying]

What I did

Added sflow dropped packet notification support, HLD : sonic-net/SONiC#1786

How I did it

Enhancement

How to verify it

Added UT to verify the commands.

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

show sflow

sFlow Global Information:
  sFlow Admin State:               up
  sFlow Sample Direction:          both
  sFlow Polling Interval:          0
  sFlow AgentID:                   default

  2 Collectors configured:
    Name: prod                     IP addr: fe80::6e82:6aff:fe1e:cd8e UDP port: 6343   VRF: mgmt
    Name: ser5                     IP addr: 172.21.35.15    UDP port: 6343   VRF: default

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

show sflow

sFlow Global Information:
  sFlow Admin State:               up
  sFlow Sample Direction:          both
  sFlow Polling Interval:          0
  sFlow Drop Notification Limit:   0
  sFlow AgentID:                   default

  2 Collectors configured:
    Name: prod                     IP addr: fe80::6e82:6aff:fe1e:cd8e UDP port: 6343   VRF: mgmt
    Name: ser5                     IP addr: 172.21.35.15    UDP port: 6343   VRF: default

[Copilot]

Pull Request Overview

This PR adds support for sFlow drop notification monitoring with rate limiting. It introduces a new CLI command to enable/disable drop monitoring and configure the maximum rate of drop notifications.

• New drop-monitor-limit command to configure drop notification rate limiting (0-500 packets/sec)
• Updated display formatting with consistent column alignment (ljust_len = 35)
• Added test coverage for the new drop-monitor-limit functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
config/main.py	Added drop_monitor_limit command and is_sflow_mirror_on_drop_supported() helper function to configure drop notification rate limiting
show/sflow.py	Updated display formatting for consistent alignment and added drop notification limit to output
tests/sflow_test.py	Added test for drop-monitor-limit command and updated expected output strings to match new formatting, fixed import path for YANG validation mocks
doc/Command-Reference.md	Added documentation for the new config sflow drop-monitor-limit command

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