Custom email: Add support for X-Delivered-To header

From #416637 (comment 1892400575)

Attempted to set up an address, e.g., project@example.org, that is an forwarding alias hosted at Gandi with destination user@example.net. SMTP server provided is handled by SparkPost.

The verification email was received successfully the target destination inbox, e.g., user@example.net. However, it seems the headers only partially match what you are expecting for the verification process, as there is no Delivered-To but instead an X-Delivered-To, namely:

• From: Provider GitLab Sender Name <project@example.org>
• X-Delivered-To: user+verify@example.net
• To: project+verify@example.org

Proposal

Add X-Delivered-To to the list of supported headers.

Implementation plan

You can use these MRs Support X-Original-To headers for email ingestion (!149874 - merged) and Resolve "Add X-Forwarded-To to allowed headers ... (!168716 - merged) as a reference. They add a new header to the list of the supported ones.

1. Add header processing to lib/gitlab/email/receiver.rb
2. Add specs to spec/lib/gitlab/email/receiver_spec.rb
3. Add documentation to doc/administration/incoming_email.md