Basically, one of the refactor tasks. In this PR, I have moved
`DelegationTokenPublisher` to the metadata module. Similar to the
`ScramPublisher` migration (commit feee50f476), I have moved
`DelegationTokenManager` to the server-common module, as it would
otherwise create a circular dependency. Moreover, I have made multiple
changes throughout the codebase to reference `DelegationTokenManager`
from server-common instead of the server module.
Reviewers: Mickael Maison <mickael.maison@gmail.com>, Chia-Ping Tsai
<chia7712@gmail.com>