Fetch incoming emails from multiple addresses

What does this MR do?

This is the first step toward having a separate incoming email address for service desk. It allows to configure mail_room to listen on multiple email addresses (but this is still deisabled - we don't add service_desk_email address into config file yet).

Once this feature is complete, user will be able to optionally configure a separate incoming email address in config/gitlab.yml, very similar to incoming email address:

 service_desk_email:
    enabled: true
    address: "user+%{key}@gmail.com"
    user: "user@gmail.com"
    password: "secret"
    host: "imap.gmail.com"
    port: 993
    ssl: true
    start_tls: false
    log_path: "log/mailroom.log"

Related to #2201 (closed)

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

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