Add support for DNSSEC validation

[mem]

The DNS client library that Blackbox Exporter uses does support DNSSEC validation (setting the DO bit), but Blackbox Exporter itself doesn't. In order to add support for this in the agent we need to:

• Add support in BBE
• Add support for requesting validation in the protobuf definition
• Add support for converting the request for validation into the corresponding setting in BBE
• Add support in the frontend for requesting validation
• Add support in the Terraform provider

[mem]

It's extra small for the agent itself, but needs work in BBE.