retrieve vendor information from DHCPv6 vendor class option (16)

Kea supports parsing vendor information for the purpose of providing vendor-specific options in the response only via vendor-specific information option (17) directly embedded in the DHCP message.

The other use case, suggested for addition in the hereby issue, is to provide vendor information via vendor class option (16) and to embed option 17 inside an option request option (6).

This could be considered as two separate features:

• look at option[16] for determining whether to send vendor options, not just for classification
• look for option[17] in ORO

One take on this would be to change the logic from:

if (option[17].exists) {
  enterprise_ID = option[17].enterprise_ID
  respond with configured vendor options
}

to:

if (option[17].exists) {
  enterprise_ID = option[17].enterprise_ID
} else if (option[6].option[17].exists && option[16].exists) {
  enterprise_ID = option[16].enterprise_ID
}
if (enterprise_ID) {
  respond with configured vendor options
}

From RFC 8415, section 21.17 (https://tools.ietf.org/html/rfc8415#section-21.17):

A client that is interested in receiving a Vendor-specific Information option:

• MUST specify the Vendor-specific Information option in an Option Request option.

See the todo:

/// @todo: We could get the vendor-id from vendor-class option (16).

RT#18187