Update dhcp validation to prevent switch from DHCPServer/DHCPRelay to DHCPDeactivated

[yanjunz97]

According to NSX change that we would not support static IP Allocation modification.
We add rules to only allow the DHCP mode switch between DHCPServer and DHCPRelay.

Testing done:

• subnetDHCPConfig.mode can switch between DHCPServer and DHCPRelay, but not between DHCPDeactivated and DHCPServer/DHCPRelay
• those 3 conditions can switch between each other: subnetDHCPConfig.mode=DHCPDeactivated, subnetDHCPConfig.mode=null, subnetDHCPConfig=null
• In case of subnetDHCPConfig.mode=null or subnetDHCPConfig=null, we cannot set subnetDHCPConfig.mode to DHCPServer or DHCPRelay

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.08%. Comparing base (ccc66e7) to head (729ce25).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #996   +/-   ##
=======================================
  Coverage   74.08%   74.08%           
=======================================
  Files         118      118           
  Lines       16362    16362           
=======================================
  Hits        12122    12122           
  Misses       3466     3466           
  Partials      774      774           

Flag	Coverage Δ
unit-tests	74.08% <ø> (ø)

Files with missing lines	Coverage Δ
pkg/apis/vpc/v1alpha1/subnet_types.go	100.00% <ø> (ø)
pkg/apis/vpc/v1alpha1/subnetset_types.go	100.00% <ø> (ø)

[lxiaopei]

LGTM

[yanjunz97]

Removed the e2e test for dhcp switch for now as the current e2e testbed does not apply crd for each run and the current e2e test for switch is not well-designed for the new case.
Will add back the e2e test with better cases after the crd files are applied the e2e testbed.

[lxiaopei]

LGTM

[timdengyun]

LGTM