Allow for automated sending of DSA/DLA message after building/autopkgtest/migration

(as discussed in the Debian LTS meeting 2025-01-23, this is an idea and not a task to be worked on without prior discussion)

Currently the upload of a package and the sending of a DSA/DLA message are done asynchronously, and thus error-prone. It's for example easy to send a DLA out when the packages are stuck in the build queue. Or you'd have to actively check the migration status before sending off the mail.

At the same time it's somewhat cumbersome, as Debian mailing lists requiring GPG signatures do not work with the most common mail client, Thunderbird.

So I propose the following: A tool that uploads the package and accepts a signed body content (the DSA/DLA), which automatically sends it off once the package has built and migrated. It should not require a locally configured MTA to function. This service could be tied in via the debusine API.