DHCPMessage: support DHCP option 'vendor specific info'

This adds the support for DHCP option 'vendor specific
information'.
The information is an opaque object of n bytes, presumably
interpreted by vendor-specific code on the clients and
servers.
While there, I add the corresonding ByteArray parser in class
DHCPOptionParser.

Bug: 25642025
TEST=compile and unittest

Change-Id: I8345e336ad0de6ba9cdd90c36ce5a416f520e789
diff --git a/dhcp_options.h b/dhcp_options.h
index 8257585..9bd7a24 100644
--- a/dhcp_options.h
+++ b/dhcp_options.h
@@ -24,6 +24,7 @@
 const uint8_t kDHCPOptionRouter = 3;
 const uint8_t kDHCPOptionDNSServer = 6;
 const uint8_t kDHCPOptionDomainName = 15;
+const uint8_t kDHCPOptionVendorSpecificInformation = 43;
 const uint8_t kDHCPOptionLeaseTime = 51;
 const uint8_t kDHCPOptionMessageType = 53;
 const uint8_t kDHCPOptionServerIdentifier = 54;