ClientId for DoH upstream

[apearson]

Longer Description

#207

Feature

A ClientId is passed to DoH upstream

How

Use DoH with ClientId in:

• Headers (exposing current behavior in ctrld)
• Subdomain (used at NextDNS / AdGuard Home)
  • Example: {clientId}.dns.com
• Path (used in NextDNS / AdGuard Home)
  • Example: https://dns.com/dns-query/{clientId}?dns=AAAAAA

Technical How

• Reuse the upstream option send_client_info to enable sending client info (id)
• Reuse the service option client_id_preference to choose what is chosen for the clientId
• New upstream option client_id_type (headers, subdomain, path) to choose where the clientId goes
• Modify the URL in the DoH resolve function
• Send ControlD headers to 3rd party resolver if client_id_type is set to headers

Notes

• There are no tests for the fork's code
• Let me know if anything is needed in this PR

Thank you,

• Andrew (@apearson)