DNS check operations

[octo-yart]

Expected features :

• have some basic tools to make technical tests on domains in src/dns
  • get dns records for the auth server
  • check the MX is valid
  • check the SPF record is present and valid
  • do some syntax check on the SPF record ?
  • check the DKIM record is present and valid
  • check the CNAME records related to the features
• in post_domain, checks the domain exists, start a background full check
• store the testing status of the domain (in sql_api ?)
• in get_domain et al., give the test results about the domain, with the errors
• create a verb to immediately start the check of a domain (sync or background?)
• periodicaly check all the domains (how? in fastapi? in a dedicated server?)
• maybe some domain provisioning activities
  • create the certificates
  • create the dkim key
  • choose the dkim selector