Add canonical version of user's primary email addresses

What does this MR do?

Add new canonical email address table to represent the normalised version (without . and anything after + in the Agent part of the address).

• Only add this UserCanonicalEmail record if the email domain matches the pattern specified.
• Destroy the UserCanonicalEmail record if the user's email is modified to a non-matching domain.
• Add (if one doesn't already exist) or update the UserCanonicalEmail record if the user's email is modified to a matching domain.

Currently limited to primary email addresses only, and limited only to @gmail.com addresses.

Related to #205383

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #205383