dhcp client: add class DHCPOptionWriter

Add class DHCPOptionsWriter for writing dhcp options.
Also add the corresponding unittests.
while there, change the variable name from 'option_number'
to 'option_code' for better readability.

Bug: 25642025
TEST=compile and unittests
Change-Id: I6410651d32525760405df427b38c961ce106293f
8 files changed