Stuart Stock

Adds a new tag `SRV` and verification process. Ref Cloudflare implementation tracking issue https://github.com/cloudflare/roughtime/issues/53 for draft10

This is the "killer feature" of roughtime and needs better support in the client. Cloudflare's [roughtime implementaion](https://github.com/cloudflare/roughtime/blob/59ac654a20f8d1406296758943d1fd63d3345c76/protocol/protocol.go#L237) can build response chains. Look at that for inspiration.