Add alternativeEmail validation and mailbox availability checks for Titan Email #5705163

Changes proposed in this Pull Request

• The alternativeEmail field is now validated against the active email domain to ensure that users do not erroneously set an email address on that domain as the alternative email address.
• Additional mailboxes are subjected to an availability check to ensure that the new mailbox names are not already taken.
• A new endpoint is added/consumed to check availabilities for new mailboxes ( works with D59619-code )

Testing instructions

• Add Email to a domain
• Attempt to add an alternate email with the same domain as the owning, active domain.
• You should see an error similar to the following …

• Ensure that you already have existing mailboxes on the domain
• Attempt to add an additional existing mailbox
• You should see an error similar to the following …