Improve SubnetDHCPConfig validation rule

[yanjunz97]

Subnet controller will update the SubnetSpec as the following when
either accessMode or ipv4SubnetSize is not provided.

spec:
  accessMode: Public
  ipv4SubnetSize: 64
  subnetDHCPConfig:{}

Thus we need another validation rule on subnetDHCPConfig to prevent
subnetDHCPConfig being updated from no mode specified to
DHCPServer/DHCPRelay mode.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.37%. Comparing base (86c0d65) to head (82d0f44).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #976   +/-   ##
=======================================
  Coverage   73.37%   73.37%           
=======================================
  Files         118      118           
  Lines       16394    16394           
=======================================
  Hits        12029    12029           
  Misses       3581     3581           
  Partials      784      784           

Flag	Coverage Δ
unit-tests	73.37% <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)