[dhcp_server] Add config_db monitor and customize options for dhcpservd

[yaqiangz]

Why I did it

Add config_db monitor and customize options for dhcpservd. HLD: sonic-net/SONiC#1282

Work item tracking

• Microsoft ADO (number only): 25600859

How I did it

1. Add support to customize unassigned DHCP options. Current support type: binary, boolean, ipv4-address, string, uint8, uint16, uint32
2. Add db config change monitor for dhcpservd

How to verify it

Unit tests in sonic-dhcp-server all passed

=============================================================================================== test session starts ===============================================================================================
platform linux -- Python 3.9.2, pytest-6.0.2, py-1.10.0, pluggy-0.13.0
rootdir: /sonic/src/sonic-dhcp-server, configfile: setup.cfg, testpaths: tests
plugins: pyfakefs-5.2.4, cov-2.10.1
collected 261 items                                                                                                                                                                                               

tests/test_dhcp_cfggen.py ..................                                                                                                                                                                [  6%]
tests/test_dhcp_db_monitor.py ............................................................................................................                                                                  [ 48%]
tests/test_dhcp_lease.py .....                                                                                                                                                                              [ 50%]
tests/test_dhcprelayd.py ........................................................................................                                                                                           [ 83%]
tests/test_dhcpservd.py .......                                                                                                                                                                             [ 86%]
tests/test_utils.py ...................................                                                                                                                                                     [100%]Clearing the cache


----------- coverage: platform linux, python 3.9.2-final-0 -----------
Name                                    Stmts   Miss Branch BrPart     Cover   Missing
--------------------------------------------------------------------------------------
dhcp_server/dhcpservd/dhcpservd.py         67      4     14      0    92.59%   89-96
dhcp_server/dhcprelayd/dhcprelayd.py      116      6     46      0    93.83%   76-82
dhcp_server/dhcpservd/dhcp_lease.py        95      3     30      4    94.40%   57->59, 59, 60->61, 61, 74->76, 112->134, 153
dhcp_server/dhcpservd/dhcp_cfggen.py      219      6     92      1    97.75%   202-206, 292->294, 294
dhcp_server/common/utils.py                69      1     38      1    98.13%   128->133, 133
dhcp_server/common/dhcp_db_monitor.py     221      0     74      1    99.66%   366->exit
--------------------------------------------------------------------------------------
TOTAL                                     787     20    294      7    96.95%

1 file skipped due to complete coverage.

Required test coverage of 80.0% reached. Total coverage: 96.95%

Signed-off-by: Yaqiang Zhu yaqiangzhu@microsoft.com

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)