Add domain verification API classes

Includes the request class sent to the verification agent and the 2
data classes returned by the to-be-added DomainVerificationManager.

Exempt-From-Owner-Approval: Already approved by owners on main branch

Bug: 163565712
CTS-Coverage-Bug: 179382047

Test: atest DomainVerificationCoreApiTest

Change-Id: If9f1b987f0d06c930f6c44b58af101b83947acb9