[dhcp_server] Add support for smart switch in dhcprelayd

[yaqiangz]

Why I did it

Add support for smart switch in dhcprelayd

Work item tracking

• Microsoft ADO (number only): 26437270

How I did it

1. Add checker support for smart switch and add related UTs
   1.1 Only enable FEATURE table checker when dhcprelayd init
   1.2 In function refresh_dhcrelay, enable or disable VLAN/MID_PLANE table checkers as needed.
   1.3 When dhcp_server feature state change, do changes for checkers:

   • disable -> enabled: Should enable DHCP_SERVER_IPV4 checker
   • enabled -> enabled: Refer to refresh_dhcrelay in 1.2
   • enabled -> disabled: Should disable
   • disabled -> disabled: not need to update

2. Extract function _proceed_with_check_res in function wait to let UT cover it.

How to verify it

UTs passed

collected 430 items                                                                                                                                                                                                                                                                      

tests/test_dhcp_cfggen.py ......................                                                                                                                                                                                                                                   [  5%]
tests/test_dhcp_db_monitor.py ...............................................................................................................................................                                                                                                      [ 38%]
tests/test_dhcp_lease.py .....                                                                                                                                                                                                                                                     [ 39%]
tests/test_dhcprelayd.py ................................................................................................................................................................                                                                                          [ 76%]
tests/test_dhcpservd.py .......                                                                                                                                                                                                                                                    [ 78%]
tests/test_smart_switch.py .......................................................                                                                                                                                                                                                 [ 91%]
tests/test_utils.py ......................................                                                                                                                                                                                                                         [100%]

----------- coverage: platform linux, python 3.9.2-final-0 -----------
Name                                      Stmts   Miss Branch BrPart     Cover   Missing
----------------------------------------------------------------------------------------
dhcp_utilities/dhcpservd/dhcpservd.py        67      4     14      0    92.59%   91-98
dhcp_utilities/dhcpservd/dhcp_lease.py       95      3     30      4    94.40%   57->59, 59, 60->61, 61, 74->76, 112->134, 153
dhcp_utilities/dhcpservd/dhcp_cfggen.py     242      6    104      1    97.98%   250-254, 342->344, 344
dhcp_utilities/common/utils.py               78      1     42      1    98.33%   129->134, 134
dhcp_utilities/dhcprelayd/dhcprelayd.py     212      3     74      1    98.60%   102->90, 119-124
----------------------------------------------------------------------------------------
TOTAL                                       984     17    358      7    98.06%

2 files skipped due to complete coverage.

Required test coverage of 80.0% reached. Total coverage: 98.06%

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[yaqiangz]

@yxieca Could you please help to merge this PR?