tests: net: dhcpv4: tests for encapsulated vendor specific option callbacks

Add tests for the encapsulated vendor specific option callback API.
These test cases are focused on correct parsing of encapsulated
vendor specific options.
The sample DHCP messages have been extended accordingly.

Signed-off-by: Matthias Breithaupt <m.breithaupt@vogl-electronic.com>
Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>
Co-authored-by: Fin Maaß <f.mass@vogl-electronic.com>